[PATCH 5/5] x86: PAT: make pat_x_mtrr_type() more readable

[PATCH 5/5] x86: PAT: make pat_x_mtrr_type() more readable

[Andreas Herrmann]

I've found it inconvenient to review pat_x_mtrr_type().
Thus I slightly changed it and added some comment to make it more
readable.
I've also added BUG statements for (some) unused/unhandled PAT/MTRR types.

Signed-off-by: Andreas Herrmann <andreas.herrmann3@amd.com>
---
 arch/x86/mm/pat.c |   30 +++++++++++++++++++++---------
 1 files changed, 21 insertions(+), 9 deletions(-)

diff --git a/arch/x86/mm/pat.c b/arch/x86/mm/pat.c
index 2f6c33d..fc0a397 100644
--- a/arch/x86/mm/pat.c
+++ b/arch/x86/mm/pat.c
@@ -185,18 +185,30 @@ static int pat_x_mtrr_type(u64 start, u64 end, unsigned long prot,
 	prot &= (~_PAGE_CACHE_MASK);
 
 	/* Currently doing intersection by hand. Optimize it later. */
-	if (pat_type == _PAGE_CACHE_WC) {
+	/*    effective type		        ret_prot
+	  pat \ mtrr  WB WC UC		pat \ mtrr  WB  WC  UC
+	  WC          WC WC WC		WC          WC  WC  WC
+	  UC-         UC WC UC		UC-         UC- UC- UC-
+	  UC          UC UC UC		UC          UC  UC  UC
+	  WB          WB WC UC		WB          WB  WC  UC
+	 */
+	if (pat_type == _PAGE_CACHE_WC)
 		*ret_prot = prot | _PAGE_CACHE_WC;
-	} else if (pat_type == _PAGE_CACHE_UC_MINUS) {
+	else if (pat_type == _PAGE_CACHE_UC_MINUS)
 		*ret_prot = prot | _PAGE_CACHE_UC_MINUS;
-	} else if (pat_type == _PAGE_CACHE_UC ||
-		   mtrr_type == MTRR_TYPE_UNCACHABLE) {
+	else if (pat_type == _PAGE_CACHE_UC)
 		*ret_prot = prot | _PAGE_CACHE_UC;
-	} else if (mtrr_type == MTRR_TYPE_WRCOMB) {
-		*ret_prot = prot | _PAGE_CACHE_WC;
-	} else {
-		*ret_prot = prot | _PAGE_CACHE_WB;
-	}
+	else if (pat_type == _PAGE_CACHE_WB) {
+		if (mtrr_type == MTRR_TYPE_WRBACK)
+			*ret_prot = prot | _PAGE_CACHE_WB;
+		else if (mtrr_type == MTRR_TYPE_WRCOMB)
+			*ret_prot = prot | _PAGE_CACHE_WC;
+		else if (mtrr_type == MTRR_TYPE_UNCACHABLE)
+			*ret_prot = prot | _PAGE_CACHE_UC;
+		else
+			BUG();
+	} else
+		  BUG();
 
 	return 0;
 }
-- 
1.5.5.3

[PATCH 5/5 v2] x86: PAT: make pat_x_mtrr_type() more readable

[Andreas Herrmann]

I've found it inconvenient to review pat_x_mtrr_type().
Thus I slightly changed it and added some comment to make
it more readable.

I've also added BUG statements for (some) unused/unhandled PAT/MTRR
types.

Signed-off-by: Andreas Herrmann <andreas.herrmann3@amd.com>
---
New patch version. (against current tip/x86/pat - v2.6.26-rc6-105-gfaeca31)
Previous version had some conflict with another commit in tip/master.

Regards,
Andreas

 arch/x86/mm/pat.c |   50 +++++++++++++++++++++++++++-----------------------
 1 files changed, 27 insertions(+), 23 deletions(-)

diff --git a/arch/x86/mm/pat.c b/arch/x86/mm/pat.c
index 4beccea..cdf2c15 100644
--- a/arch/x86/mm/pat.c
+++ b/arch/x86/mm/pat.c
@@ -169,35 +169,39 @@ static int pat_x_mtrr_type(u64 start, u64 end, unsigned long prot,
 	prot &= (~_PAGE_CACHE_MASK);
 
 	/*
-	 * We return the PAT request directly for types where PAT takes
-	 * precedence with respect to MTRR and for UC_MINUS.
+	 * We return the PAT request directly for types where PAT
+	 * takes precedence with respect to MTRR and for UC_MINUS. In
+	 * case of pat_type==WB we have to know mtrr_type to get the
+	 * effective type.
+	 *
+	 *    effective type                ret_prot
+	 *  pat \ mtrr  WB WC UC    pat \ mtrr  WB  WC  UC
+	 *  WC          WC WC WC    WC          WC  WC  WC
+	 *  UC-         UC WC UC    UC-         UC- UC- UC-
+	 *  UC          UC UC UC    UC          UC  UC  UC
+	 *  WB          WB WC UC    WB          WB  WC  UC
+	 *
 	 * Consistency checks with other PAT requests is done later
 	 * while going through memtype list.
 	 */
-	if (pat_type == _PAGE_CACHE_WC) {
+	if (pat_type == _PAGE_CACHE_WC)
 		*ret_prot = prot | _PAGE_CACHE_WC;
-		return 0;
-	} else if (pat_type == _PAGE_CACHE_UC_MINUS) {
+	else if (pat_type == _PAGE_CACHE_UC_MINUS)
 		*ret_prot = prot | _PAGE_CACHE_UC_MINUS;
-		return 0;
-	} else if (pat_type == _PAGE_CACHE_UC) {
-		*ret_prot = prot | _PAGE_CACHE_UC;
-		return 0;
-	}
-
-	/*
-	 * Look for MTRR hint to get the effective type in case where PAT
-	 * request is for WB.
-	 */
-	mtrr_type = mtrr_type_lookup(start, end);
-
-	if (mtrr_type == MTRR_TYPE_UNCACHABLE) {
+	else if (pat_type == _PAGE_CACHE_UC)
 		*ret_prot = prot | _PAGE_CACHE_UC;
-	} else if (mtrr_type == MTRR_TYPE_WRCOMB) {
-		*ret_prot = prot | _PAGE_CACHE_WC;
-	} else {
-		*ret_prot = prot | _PAGE_CACHE_WB;
-	}
+	else if (pat_type == _PAGE_CACHE_WB) {
+		mtrr_type = mtrr_type_lookup(start, end);
+		if (mtrr_type == MTRR_TYPE_WRBACK)
+			*ret_prot = prot | _PAGE_CACHE_WB;
+		else if (mtrr_type == MTRR_TYPE_WRCOMB)
+			*ret_prot = prot | _PAGE_CACHE_WC;
+		else if (mtrr_type == MTRR_TYPE_UNCACHABLE)
+			*ret_prot = prot | _PAGE_CACHE_UC;
+		else
+			BUG();
+	} else
+		  BUG();
 
 	return 0;
 }
-- 
1.5.5.4

Re: [PATCH 5/5 v2] x86: PAT: make pat_x_mtrr_type() more readable

[Hugh Dickins]

On Mon, 16 Jun 2008, Andreas Herrmann wrote:
> I've found it inconvenient to review pat_x_mtrr_type().
> Thus I slightly changed it and added some comment to make
> it more readable.

I found it hard to read too; but it's amusing how utterly different
are our ideas to make it more readable.  Most of what it is doing
seems to me confusingly left over from earlier implementations.

I've appended my version at the bottom: my involvement in pat.c is
not very strong, so would you like to take over my patch and combine
best of both into one?  I don't particularly want to stroll along
after, undoing what you did.

> 
> I've also added BUG statements for (some) unused/unhandled PAT/MTRR
> types.

I suspect your pat_type BUG is uninteresting (given _PAGE_CACHE_MASK
only has two bits to play with), and your mtrr_type BUG dangerous -
mtrr_type_lookup may be returning other values, which the current
code tolerates, I believe intentionally; but perhaps you've read
further than I did, and convinced yourself they cannot get there.

> 
> Signed-off-by: Andreas Herrmann <andreas.herrmann3@amd.com>
> ---
> New patch version. (against current tip/x86/pat - v2.6.26-rc6-105-gfaeca31)
> Previous version had some conflict with another commit in tip/master.
> 
> Regards,
> Andreas
> 
>  arch/x86/mm/pat.c |   50 +++++++++++++++++++++++++++-----------------------
>  1 files changed, 27 insertions(+), 23 deletions(-)
> 
> diff --git a/arch/x86/mm/pat.c b/arch/x86/mm/pat.c
> index 4beccea..cdf2c15 100644
> --- a/arch/x86/mm/pat.c
> +++ b/arch/x86/mm/pat.c
> @@ -169,35 +169,39 @@ static int pat_x_mtrr_type(u64 start, u64 end, unsigned long prot,
>  	prot &= (~_PAGE_CACHE_MASK);
>  
>  	/*
> -	 * We return the PAT request directly for types where PAT takes
> -	 * precedence with respect to MTRR and for UC_MINUS.
> +	 * We return the PAT request directly for types where PAT
> +	 * takes precedence with respect to MTRR and for UC_MINUS. In
> +	 * case of pat_type==WB we have to know mtrr_type to get the
> +	 * effective type.
> +	 *
> +	 *    effective type                ret_prot
> +	 *  pat \ mtrr  WB WC UC    pat \ mtrr  WB  WC  UC
> +	 *  WC          WC WC WC    WC          WC  WC  WC
> +	 *  UC-         UC WC UC    UC-         UC- UC- UC-
> +	 *  UC          UC UC UC    UC          UC  UC  UC
> +	 *  WB          WB WC UC    WB          WB  WC  UC

I'm inclined to think that once the obfuscations in the code are
removed, the code becomes easier to understand than your comment
table (which, to be honest, I haven't even checked through).

> +	 *
>  	 * Consistency checks with other PAT requests is done later
>  	 * while going through memtype list.
>  	 */
> -	if (pat_type == _PAGE_CACHE_WC) {
> +	if (pat_type == _PAGE_CACHE_WC)
>  		*ret_prot = prot | _PAGE_CACHE_WC;
> -		return 0;
> -	} else if (pat_type == _PAGE_CACHE_UC_MINUS) {
> +	else if (pat_type == _PAGE_CACHE_UC_MINUS)
>  		*ret_prot = prot | _PAGE_CACHE_UC_MINUS;
> -		return 0;
> -	} else if (pat_type == _PAGE_CACHE_UC) {
> -		*ret_prot = prot | _PAGE_CACHE_UC;
> -		return 0;
> -	}
> -
> -	/*
> -	 * Look for MTRR hint to get the effective type in case where PAT
> -	 * request is for WB.
> -	 */
> -	mtrr_type = mtrr_type_lookup(start, end);
> -
> -	if (mtrr_type == MTRR_TYPE_UNCACHABLE) {
> +	else if (pat_type == _PAGE_CACHE_UC)
>  		*ret_prot = prot | _PAGE_CACHE_UC;
> -	} else if (mtrr_type == MTRR_TYPE_WRCOMB) {
> -		*ret_prot = prot | _PAGE_CACHE_WC;
> -	} else {
> -		*ret_prot = prot | _PAGE_CACHE_WB;
> -	}
> +	else if (pat_type == _PAGE_CACHE_WB) {
> +		mtrr_type = mtrr_type_lookup(start, end);
> +		if (mtrr_type == MTRR_TYPE_WRBACK)
> +			*ret_prot = prot | _PAGE_CACHE_WB;
> +		else if (mtrr_type == MTRR_TYPE_WRCOMB)
> +			*ret_prot = prot | _PAGE_CACHE_WC;
> +		else if (mtrr_type == MTRR_TYPE_UNCACHABLE)
> +			*ret_prot = prot | _PAGE_CACHE_UC;
> +		else
> +			BUG();
> +	} else
> +		  BUG();
>  
>  	return 0;
>  }

Clean up over-complications in pat_x_mtrr_type().
And if reserve_memtype() ignores stray req_type bits when
pat_enabled, it's better to mask them off when not also.

Signed-off-by: Hugh Dickins <hugh@veritas.com>
---

 arch/x86/mm/pat.c |   47 +++++++++++---------------------------------
 1 file changed, 12 insertions(+), 35 deletions(-)

--- a/arch/x86/mm/pat.c
+++ b/arch/x86/mm/pat.c
@@ -145,47 +145,31 @@ static DEFINE_SPINLOCK(memtype_lock); 	/
  * The intersection is based on "Effective Memory Type" tables in IA-32
  * SDM vol 3a
  */
-static int pat_x_mtrr_type(u64 start, u64 end, unsigned long prot,
-				unsigned long *ret_prot)
+static unsigned long pat_x_mtrr_type(u64 start, u64 end, unsigned long req_type)
 {
-	unsigned long pat_type;
 	u8 mtrr_type;
 
-	pat_type = prot & _PAGE_CACHE_MASK;
-	prot &= (~_PAGE_CACHE_MASK);
-
 	/*
 	 * We return the PAT request directly for types where PAT takes
 	 * precedence with respect to MTRR and for UC_MINUS.
 	 * Consistency checks with other PAT requests is done later
 	 * while going through memtype list.
 	 */
-	if (pat_type == _PAGE_CACHE_WC) {
-		*ret_prot = prot | _PAGE_CACHE_WC;
-		return 0;
-	} else if (pat_type == _PAGE_CACHE_UC_MINUS) {
-		*ret_prot = prot | _PAGE_CACHE_UC_MINUS;
-		return 0;
-	} else if (pat_type == _PAGE_CACHE_UC) {
-		*ret_prot = prot | _PAGE_CACHE_UC;
-		return 0;
-	}
+	if (req_type == _PAGE_CACHE_WC ||
+	    req_type == _PAGE_CACHE_UC_MINUS ||
+	    req_type == _PAGE_CACHE_UC)
+		return req_type;
 
 	/*
 	 * Look for MTRR hint to get the effective type in case where PAT
 	 * request is for WB.
 	 */
 	mtrr_type = mtrr_type_lookup(start, end);
-
-	if (mtrr_type == MTRR_TYPE_UNCACHABLE) {
-		*ret_prot = prot | _PAGE_CACHE_UC;
-	} else if (mtrr_type == MTRR_TYPE_WRCOMB) {
-		*ret_prot = prot | _PAGE_CACHE_WC;
-	} else {
-		*ret_prot = prot | _PAGE_CACHE_WB;
-	}
-
-	return 0;
+	if (mtrr_type == MTRR_TYPE_UNCACHABLE)
+		return _PAGE_CACHE_UC;
+	if (mtrr_type == MTRR_TYPE_WRCOMB)
+		return _PAGE_CACHE_WC;
+	return _PAGE_CACHE_WB;
 }
 
 /*
@@ -218,7 +202,7 @@ int reserve_memtype(u64 start, u64 end, 
 			if (req_type == -1) {
 				*ret_type = _PAGE_CACHE_WB;
 			} else {
-				*ret_type = req_type;
+				*ret_type = req_type & _PAGE_CACHE_MASK;
 			}
 		}
 		return 0;
@@ -250,14 +234,7 @@ int reserve_memtype(u64 start, u64 end, 
 		}
 	} else {
 		req_type &= _PAGE_CACHE_MASK;
-		err = pat_x_mtrr_type(start, end, req_type, &actual_type);
-	}
-
-	if (err) {
-		if (ret_type)
-			*ret_type = actual_type;
-
-		return -EINVAL;
+		actual_type = pat_x_mtrr_type(start, end, req_type);
 	}
 
 	new_entry  = kmalloc(sizeof(struct memtype), GFP_KERNEL);

Re: [PATCH 5/5 v2] x86: PAT: make pat_x_mtrr_type() more readable

[Ingo Molnar]

* Hugh Dickins <hugh@veritas.com> wrote:

> Clean up over-complications in pat_x_mtrr_type(). And if 
> reserve_memtype() ignores stray req_type bits when pat_enabled, it's 
> better to mask them off when not also.

applied, to tip/x86/pat, thanks Hugh. Andreas, if there's any other 
clean up you can think of here please send a delta patch.

	Ingo

Re: [PATCH 5/5 v2] x86: PAT: make pat_x_mtrr_type() more readable

[Andreas Herrmann]

On Mon, Jun 16, 2008 at 06:42:43PM +0100, Hugh Dickins wrote:
> On Mon, 16 Jun 2008, Andreas Herrmann wrote:
> > I've found it inconvenient to review pat_x_mtrr_type().
> > Thus I slightly changed it and added some comment to make
> > it more readable.
> 
> I found it hard to read too; but it's amusing how utterly different
> are our ideas to make it more readable.  Most of what it is doing
> seems to me confusingly left over from earlier implementations.

;-)

> I've appended my version at the bottom: my involvement in pat.c is
> not very strong, so would you like to take over my patch and combine
> best of both into one?  I don't particularly want to stroll along
> after, undoing what you did.

Why combining both patches?
I've checked your patch and found it more straightforward than mine.
And the attached patch makes it even shorter.
(patch against tip/x86/pat -- where your patch is already residing)

> > 
> > I've also added BUG statements for (some) unused/unhandled PAT/MTRR
> > types.
> 
> I suspect your pat_type BUG is uninteresting (given _PAGE_CACHE_MASK

It's just interesting if someone would change that mask and forget to
handle potential new pat_types. I admit that is rather unlikely.

> only has two bits to play with), and your mtrr_type BUG dangerous -

Well, the former code had this snippet in pat_x_mtrr_type():

-       mtrr_type = mtrr_type_lookup(start, end);
-       if (mtrr_type == 0xFF) {                /* MTRR not enabled */
-               *ret_prot = prot;
-               return 0;
-       }
-       if (mtrr_type == 0xFE) {                /* MTRR match error */
-               *ret_prot = _PAGE_CACHE_UC;
-               return -1;
-       }
-       if (mtrr_type != MTRR_TYPE_UNCACHABLE &&
-           mtrr_type != MTRR_TYPE_WRBACK &&
-           mtrr_type != MTRR_TYPE_WRCOMB) {    /* MTRR type unhandled */
-               *ret_prot = _PAGE_CACHE_UC;
-               return -1;
-       }
-

But now it seems that the function intentional handles
MTRR_TYPE_WRTHROUGH and the 0xFE/0xFF cases and thus the BUG statement
is wrong.

Thanks,

Andreas

---
[PATCH] x86: shrink pat_x_mtrr_type to its essentials

Signed-off-by: Andreas Herrmann <andreas.herrmann3@amd.com>
---
 arch/x86/mm/pat.c |   30 +++++++++++-------------------
 1 files changed, 11 insertions(+), 19 deletions(-)

diff --git a/arch/x86/mm/pat.c b/arch/x86/mm/pat.c
index ac3a2b1..227df3c 100644
--- a/arch/x86/mm/pat.c
+++ b/arch/x86/mm/pat.c
@@ -161,29 +161,21 @@ static DEFINE_SPINLOCK(memtype_lock); 	/* protects memtype list */
  */
 static unsigned long pat_x_mtrr_type(u64 start, u64 end, unsigned long req_type)
 {
-	u8 mtrr_type;
-
-	/*
-	 * We return the PAT request directly for types where PAT takes
-	 * precedence with respect to MTRR and for UC_MINUS.
-	 * Consistency checks with other PAT requests is done later
-	 * while going through memtype list.
-	 */
-	if (req_type == _PAGE_CACHE_WC ||
-	    req_type == _PAGE_CACHE_UC_MINUS ||
-	    req_type == _PAGE_CACHE_UC)
-		return req_type;
-
 	/*
 	 * Look for MTRR hint to get the effective type in case where PAT
 	 * request is for WB.
 	 */
-	mtrr_type = mtrr_type_lookup(start, end);
-	if (mtrr_type == MTRR_TYPE_UNCACHABLE)
-		return _PAGE_CACHE_UC;
-	if (mtrr_type == MTRR_TYPE_WRCOMB)
-		return _PAGE_CACHE_WC;
-	return _PAGE_CACHE_WB;
+	if (req_type == _PAGE_CACHE_WB) {
+		u8 mtrr_type;
+
+		mtrr_type = mtrr_type_lookup(start, end);
+		if (mtrr_type == MTRR_TYPE_UNCACHABLE)
+			return _PAGE_CACHE_UC;
+		if (mtrr_type == MTRR_TYPE_WRCOMB)
+			return _PAGE_CACHE_WC;
+	}
+
+	return req_type;
 }
 
 /*
-- 
1.5.5.4

Re: [PATCH 5/5 v2] x86: PAT: make pat_x_mtrr_type() more readable

[Hugh Dickins]

On Wed, 18 Jun 2008, Andreas Herrmann wrote:
> And the attached patch makes it even shorter.
> (patch against tip/x86/pat -- where your patch is already residing)

Yes, that's delightful, even better, thanks.  After accusing you
of an uninteresting BUG on a fifth value from 2 bits, I had an
inkling that maybe my patch was therefore still stupidly verbose.

> ---
> [PATCH] x86: shrink pat_x_mtrr_type to its essentials
> 
> Signed-off-by: Andreas Herrmann <andreas.herrmann3@amd.com>

Acked-by: Hugh Dickins <hugh@veritas.com>

> ---
>  arch/x86/mm/pat.c |   30 +++++++++++-------------------
>  1 files changed, 11 insertions(+), 19 deletions(-)
> 
> diff --git a/arch/x86/mm/pat.c b/arch/x86/mm/pat.c
> index ac3a2b1..227df3c 100644
> --- a/arch/x86/mm/pat.c
> +++ b/arch/x86/mm/pat.c
> @@ -161,29 +161,21 @@ static DEFINE_SPINLOCK(memtype_lock); 	/* protects memtype list */
>   */
>  static unsigned long pat_x_mtrr_type(u64 start, u64 end, unsigned long req_type)
>  {
> -	u8 mtrr_type;
> -
> -	/*
> -	 * We return the PAT request directly for types where PAT takes
> -	 * precedence with respect to MTRR and for UC_MINUS.
> -	 * Consistency checks with other PAT requests is done later
> -	 * while going through memtype list.
> -	 */
> -	if (req_type == _PAGE_CACHE_WC ||
> -	    req_type == _PAGE_CACHE_UC_MINUS ||
> -	    req_type == _PAGE_CACHE_UC)
> -		return req_type;
> -
>  	/*
>  	 * Look for MTRR hint to get the effective type in case where PAT
>  	 * request is for WB.
>  	 */
> -	mtrr_type = mtrr_type_lookup(start, end);
> -	if (mtrr_type == MTRR_TYPE_UNCACHABLE)
> -		return _PAGE_CACHE_UC;
> -	if (mtrr_type == MTRR_TYPE_WRCOMB)
> -		return _PAGE_CACHE_WC;
> -	return _PAGE_CACHE_WB;
> +	if (req_type == _PAGE_CACHE_WB) {
> +		u8 mtrr_type;
> +
> +		mtrr_type = mtrr_type_lookup(start, end);
> +		if (mtrr_type == MTRR_TYPE_UNCACHABLE)
> +			return _PAGE_CACHE_UC;
> +		if (mtrr_type == MTRR_TYPE_WRCOMB)
> +			return _PAGE_CACHE_WC;
> +	}
> +
> +	return req_type;
>  }
>  
>  /*

Re: [PATCH 5/5 v2] x86: PAT: make pat_x_mtrr_type() more readable

[Ingo Molnar]

* Andreas Herrmann <andreas.herrmann3@amd.com> wrote:

> [PATCH] x86: shrink pat_x_mtrr_type to its essentials
> 
> Signed-off-by: Andreas Herrmann <andreas.herrmann3@amd.com>

applied to tip/x86/pat - thanks Andreas.

now pat_x_mtrr_type() is not nearly as scary anymore :-)

	Ingo