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Re: [Xen-devel] [PATCH] x86: don't write_tsc() non-zero values on CPUs u

To: "Keir Fraser" <keir.xen@xxxxxxxxx>
Subject: Re: [Xen-devel] [PATCH] x86: don't write_tsc() non-zero values on CPUs updating only the lower 32 bits
From: "Jan Beulich" <JBeulich@xxxxxxxxxx>
Date: Thu, 14 Apr 2011 08:42:07 +0100
Cc: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>, "winston.l.wang" <winston.l.wang@xxxxxxxxx>
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>>> On 14.04.11 at 09:25, Keir Fraser <keir.xen@xxxxxxxxx> wrote:
> On 14/04/2011 08:18, "Jan Beulich" <JBeulich@xxxxxxxxxx> wrote:
>> This means suppressing the uses in time_calibration_tsc_rendezvous(),
>> cstate_restore_tsc(), and synchronize_tsc_slave(), and fixes a boot
>> hang of Linux Dom0 when loading processor.ko on such systems that
>> have support for C states above C1.
> Should your new test be gated on !X86_FEATURE_TSC_RELIABLE? We already

Which test? The write-TSC-probe itself?

> *never* write the TSC when boot_cpu_has(TSC_RELIABLE) -- Dan Magenheimer
> made that change on the assumption that TSCs were globally synced by
> firmware in this case, and us writing one or more TSCs could only ever make
> things worse.

That's not true - we only avoid the writing for TSC sync during boot.
Post-boot bringup of CPUs will write the TSC no matter what, and
cstate_restore_tsc() also has no such gating afaics.


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