[PATCH] x86/tsc: fix 64bit divisor be truncated in calc_hpet_ref

[PATCH] x86/tsc: fix 64bit divisor be truncated in calc_hpet_ref

[Xiaoming Gao]

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 From ba3d2fb699c4d8ee61b05d7e70be48b9c4e22baf Mon Sep 17 00:00:00 2001
From: Xiaoming Gao <newtongao@tencent.com>
Date: Fri, 13 Apr 2018 17:05:18 +0800
Subject: [PATCH] x86/tsc: fix 64bit divisor be truncated in calc_hpet_ref

the HPET frequency got larger on intel skylake, thus could cause tmp to
exceed 32bits.
do_div will truncate 64bits tmp to 32bits, so the frequency calced via
HPET will be wrong, use div64_u64 can fix it.

Signed-off-by: Xiaoming Gao <newtongao@tencent.com>
---
  arch/x86/kernel/tsc.c |    2 +-
  1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index 9714a7a..8700269 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -160,7 +160,7 @@ static unsigned long calc_hpet_ref(u64 deltatsc, u64 
hpet1, u64 hpet2)
      hpet2 -= hpet1;
      tmp = ((u64)hpet2 * hpet_readl(HPET_PERIOD));
      do_div(tmp, 1000000);
-    do_div(deltatsc, tmp);
+    deltatsc = div64_u64(deltatsc, tmp);

      return (unsigned long) deltatsc;
  }
-- 
1.7.1



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From ba3d2fb699c4d8ee61b05d7e70be48b9c4e22baf Mon Sep 17 00:00:00 2001
From: Xiaoming Gao <newtongao@tencent.com>
Date: Fri, 13 Apr 2018 17:05:18 +0800
Subject: [PATCH] x86/tsc: fix 64bit divisor be truncated in calc_hpet_ref

the HPET frequency got larger on intel skylake, thus could cause tmp to
exceed 32bits.
do_div will truncate 64bits tmp to 32bits, so the frequency calced via
HPET will be wrong, use div64_u64 can fix it.

Signed-off-by: Xiaoming Gao <newtongao@tencent.com>
---
 arch/x86/kernel/tsc.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index 9714a7a..8700269 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -160,7 +160,7 @@ static unsigned long calc_hpet_ref(u64 deltatsc, u64 hpet1, u64 hpet2)
 	hpet2 -= hpet1;
 	tmp = ((u64)hpet2 * hpet_readl(HPET_PERIOD));
 	do_div(tmp, 1000000);
-	do_div(deltatsc, tmp);
+	deltatsc = div64_u64(deltatsc, tmp);
 
 	return (unsigned long) deltatsc;
 }
-- 
1.7.1

Re: [PATCH] x86/tsc: fix 64bit divisor be truncated in calc_hpet_ref

[Thomas Gleixner]

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On Fri, 13 Apr 2018, Xiaoming Gao wrote:
> From ba3d2fb699c4d8ee61b05d7e70be48b9c4e22baf Mon Sep 17 00:00:00 2001
> From: Xiaoming Gao <newtongao@tencent.com>
> Date: Fri, 13 Apr 2018 17:05:18 +0800
> Subject: [PATCH] x86/tsc: fix 64bit divisor be truncated in calc_hpet_ref
> 
> the HPET frequency got larger on intel skylake, thus could cause tmp to
> exceed 32bits.
> do_div will truncate 64bits tmp to 32bits, so the frequency calced via
> HPET will be wrong, use div64_u64 can fix it.
> 
> Signed-off-by: Xiaoming Gao <newtongao@tencent.com>
> ---
>  arch/x86/kernel/tsc.c |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
> index 9714a7a..8700269 100644
> --- a/arch/x86/kernel/tsc.c
> +++ b/arch/x86/kernel/tsc.c
> @@ -160,7 +160,7 @@ static unsigned long calc_hpet_ref(u64 deltatsc, u64
> hpet1, u64 hpet2)
>      hpet2 -= hpet1;
>      tmp = ((u64)hpet2 * hpet_readl(HPET_PERIOD));
>      do_div(tmp, 1000000);
> -    do_div(deltatsc, tmp);
> +    deltatsc = div64_u64(deltatsc, tmp);
> 
>      return (unsigned long) deltatsc;

Please fix your MUA so it does not whitespace damage
patches. Documentation/process/email-clients.rst tells you how to teach
thunderbird to do the right thing.

Attaching the patch does not help either as it breaks the workflow. I've
fixed it up manually this time, but please be more careful next time.

Thanks,

	tglx

[tip:x86/urgent] x86/tsc: Prevent 32bit truncation in calc_hpet_ref()

[tip-bot for Xiaoming Gao]

Commit-ID:  d3878e164dcd3925a237a20e879432400e369172
Gitweb:     https://git.kernel.org/tip/d3878e164dcd3925a237a20e879432400e369172
Author:     Xiaoming Gao <gxm.linux.kernel@gmail.com>
AuthorDate: Fri, 13 Apr 2018 17:48:08 +0800
Committer:  Thomas Gleixner <tglx@linutronix.de>
CommitDate: Tue, 17 Apr 2018 11:50:42 +0200

x86/tsc: Prevent 32bit truncation in calc_hpet_ref()

The TSC calibration code uses HPET as reference. The conversion normalizes
the delta of two HPET timestamps:

    hpetref = ((tshpet1 - tshpet2) * HPET_PERIOD) / 1e6

and then divides the normalized delta of the corresponding TSC timestamps
by the result to calulate the TSC frequency.

    tscfreq = ((tstsc1 - tstsc2 ) * 1e6) / hpetref

This uses do_div() which takes an u32 as the divisor, which worked so far
because the HPET frequency was low enough that 'hpetref' never exceeded
32bit.

On Skylake machines the HPET frequency increased so 'hpetref' can exceed
32bit. do_div() truncates the divisor, which causes the calibration to
fail.

Use div64_u64() to avoid the problem.

[ tglx: Fixes whitespace mangled patch and rewrote changelog ]

Signed-off-by: Xiaoming Gao <newtongao@tencent.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org
Cc: peterz@infradead.org
Cc: hpa@zytor.com
Link: https://lkml.kernel.org/r/38894564-4fc9-b8ec-353f-de702839e44e@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 ef32297ff17e..91e6da48cbb6 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -317,7 +317,7 @@ static unsigned long calc_hpet_ref(u64 deltatsc, u64 hpet1, u64 hpet2)
 	hpet2 -= hpet1;
 	tmp = ((u64)hpet2 * hpet_readl(HPET_PERIOD));
 	do_div(tmp, 1000000);
-	do_div(deltatsc, tmp);
+	deltatsc = div64_u64(deltatsc, tmp);
 
 	return (unsigned long) deltatsc;
 }