[PATCH] x86, tsc: Fix tsc ratio calibration to avoid broken mdelay

[PATCH] x86, tsc: Fix tsc ratio calibration to avoid broken mdelay

[yu.c.chen]

From: Chen Yu <yu.c.chen@intel.com>

Currently we fetch the tsc radio by:
ratio = (lo >> 8) & 0x1f;
thus get bit8~bit12 of the MSR_PLATFORM_INFO, however according
to Intel 64 and IA-32 Architectures Software Developer Manual 35.5,
the ratio bit should be bit8~bit15, otherwise we might get incorrect
tsc ratio and cause system hang later(mdelay corrupted).

Fix this problem by masking 0xff instead.

Cc: 3.14+ <stable@vger.kernel.org> # 3.14+
Signed-off-by: Chen Yu <yu.c.chen@intel.com>
---
 arch/x86/kernel/tsc_msr.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kernel/tsc_msr.c b/arch/x86/kernel/tsc_msr.c
index 92ae6ac..6aa0f4d 100644
--- a/arch/x86/kernel/tsc_msr.c
+++ b/arch/x86/kernel/tsc_msr.c
@@ -92,7 +92,7 @@ unsigned long try_msr_calibrate_tsc(void)
 
 	if (freq_desc_tables[cpu_index].msr_plat) {
 		rdmsr(MSR_PLATFORM_INFO, lo, hi);
-		ratio = (lo >> 8) & 0x1f;
+		ratio = (lo >> 8) & 0xff;
 	} else {
 		rdmsr(MSR_IA32_PERF_STATUS, lo, hi);
 		ratio = (hi >> 8) & 0x1f;
-- 
2.7.4

Re: [PATCH] x86, tsc: Fix tsc ratio calibration to avoid broken mdelay

[Thomas Gleixner]

On Fri, 6 May 2016, yu.c.chen@intel.com wrote:
> From: Chen Yu <yu.c.chen@intel.com>
> 
> Currently we fetch the tsc radio by:
> ratio = (lo >> 8) & 0x1f;
> thus get bit8~bit12 of the MSR_PLATFORM_INFO, however according
> to Intel 64 and IA-32 Architectures Software Developer Manual 35.5,
> the ratio bit should be bit8~bit15, otherwise we might get incorrect
> tsc ratio and cause system hang later(mdelay corrupted).

The resulting issue is that both TSC frequency, which is used for udelay, and
the lapic timer frequency are wrong. mdelay is just the visible damage caused
by that.
 
Thanks,

	tglx

Re: [PATCH] x86, tsc: Fix tsc ratio calibration to avoid broken mdelay

[Thomas Gleixner]

On Fri, 6 May 2016, Thomas Gleixner wrote:
> On Fri, 6 May 2016, yu.c.chen@intel.com wrote:
> > From: Chen Yu <yu.c.chen@intel.com>
> > 
> > Currently we fetch the tsc radio by:
> > ratio = (lo >> 8) & 0x1f;
> > thus get bit8~bit12 of the MSR_PLATFORM_INFO, however according
> > to Intel 64 and IA-32 Architectures Software Developer Manual 35.5,
> > the ratio bit should be bit8~bit15, otherwise we might get incorrect
> > tsc ratio and cause system hang later(mdelay corrupted).
> 
> The resulting issue is that both TSC frequency, which is used for udelay, and
> the lapic timer frequency are wrong. mdelay is just the visible damage caused
> by that.

Aside of that:

> Cc: 3.14+ <stable@vger.kernel.org> # 3.14+

Please use:

Fixes: commit '....'

next time, which identifies the kernel version to which this needs to be
backported and gives a reference to the commit which caused the issue.

Thanks,

	tglx

RE: [PATCH] x86, tsc: Fix tsc ratio calibration to avoid broken mdelay

[Chen, Yu C]

> -----Original Message-----
> From: Thomas Gleixner [mailto:tglx@linutronix.de]
> Sent: Friday, May 06, 2016 5:53 PM
> To: Chen, Yu C
> Cc: x86@kernel.org; linux-kernel@vger.kernel.org; Ingo Molnar; H. Peter Anvin;
> Gao, Bin; Len Brown; Rafael J. Wysocki; 3 . 14+ # 3 . 14+
> Subject: Re: [PATCH] x86, tsc: Fix tsc ratio calibration to avoid broken mdelay
> 
> 
> On Fri, 6 May 2016, Thomas Gleixner wrote:
> > On Fri, 6 May 2016, yu.c.chen@intel.com wrote:
> > > From: Chen Yu <yu.c.chen@intel.com>
> > >
> > > Currently we fetch the tsc radio by:
> > > ratio = (lo >> 8) & 0x1f;
> > > thus get bit8~bit12 of the MSR_PLATFORM_INFO, however according to
> > > Intel 64 and IA-32 Architectures Software Developer Manual 35.5, the
> > > ratio bit should be bit8~bit15, otherwise we might get incorrect tsc
> > > ratio and cause system hang later(mdelay corrupted).
> >
> > The resulting issue is that both TSC frequency, which is used for
> > udelay, and the lapic timer frequency are wrong. mdelay is just the
> > visible damage caused by that.
> 
> Aside of that:
> 
> > Cc: 3.14+ <stable@vger.kernel.org> # 3.14+
> 
> Please use:
> 
> Fixes: commit '....'
> 
> next time, which identifies the kernel version to which this needs to be
> backported and gives a reference to the commit which caused the issue.
> 
OK, thanks!