[PATCH 1/3] x86/tsc: Use div64_ul instead of do_div()

[PATCH 1/3] x86/tsc: Use div64_ul instead of do_div()

[Jules Irenge]

do_div() truncates a u64 divisor to 32 bit.
This can lead to non-zero being truncated to zero for division.

Fix coccinelle warning
WARNING: do_div() does a 64-by-32 division, please consider using div64_ul instead

Signed-off-by: Jules Irenge <jbi.octave@gmail.com>
---
 arch/x86/kernel/tsc.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index 5a69a49acc96..2da37f33dd17 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -484,7 +484,7 @@ static unsigned long pit_calibrate_tsc(u32 latch, unsigned long ms, int loopmin)
 
 	/* Calculate the PIT value */
 	delta = t2 - t1;
-	do_div(delta, ms);
+	div64_ul(delta, ms);
 	return delta;
 }
 
-- 
2.43.2

Re: [PATCH 1/3] x86/tsc: Use div64_ul instead of do_div()

[H. Peter Anvin]

On April 28, 2024 10:31:16 AM PDT, Jules Irenge <jbi.octave@gmail.com> wrote:
>do_div() truncates a u64 divisor to 32 bit.
>This can lead to non-zero being truncated to zero for division.
>
>Fix coccinelle warning
>WARNING: do_div() does a 64-by-32 division, please consider using div64_ul instead
>
>Signed-off-by: Jules Irenge <jbi.octave@gmail.com>
>---
> arch/x86/kernel/tsc.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
>diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
>index 5a69a49acc96..2da37f33dd17 100644
>--- a/arch/x86/kernel/tsc.c
>+++ b/arch/x86/kernel/tsc.c
>@@ -484,7 +484,7 @@ static unsigned long pit_calibrate_tsc(u32 latch, unsigned long ms, int loopmin)
> 
> 	/* Calculate the PIT value */
> 	delta = t2 - t1;
>-	do_div(delta, ms);
>+	div64_ul(delta, ms);
> 	return delta;
> }
> 

Are you sure this is not a false positive? This is a *much* more expensive operation on 32 bits.

RE: [PATCH 1/3] x86/tsc: Use div64_ul instead of do_div()

[David Laight]

From: H. Peter Anvin 
> Sent: 28 April 2024 20:13
> 
> On April 28, 2024 10:31:16 AM PDT, Jules Irenge <jbi.octave@gmail.com> wrote:
> >do_div() truncates a u64 divisor to 32 bit.
> >This can lead to non-zero being truncated to zero for division.
> >
> >Fix coccinelle warning
> >WARNING: do_div() does a 64-by-32 division, please consider using div64_ul instead
> >
> >Signed-off-by: Jules Irenge <jbi.octave@gmail.com>
> >---
> > arch/x86/kernel/tsc.c | 2 +-
> > 1 file changed, 1 insertion(+), 1 deletion(-)
> >
> >diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
> >index 5a69a49acc96..2da37f33dd17 100644
> >--- a/arch/x86/kernel/tsc.c
> >+++ b/arch/x86/kernel/tsc.c
> >@@ -484,7 +484,7 @@ static unsigned long pit_calibrate_tsc(u32 latch, unsigned long ms, int loopmin)
> >
> > 	/* Calculate the PIT value */
> > 	delta = t2 - t1;
> >-	do_div(delta, ms);
> >+	div64_ul(delta, ms);
> > 	return delta;
> > }
> >
> 
> Are you sure this is not a false positive? This is a *much* more expensive operation on 32 bits.

IIRC do_div() isn't that great either.
Somewhere it changed from being a simple wrapper on the x86 'div' instruction
(ie 64 by 32 giving 32 and 32) to being able to generate a 64bit quotent.

But the patch is entirely broken as well.

	David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)