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

To: Jan Beulich <JBeulich@xxxxxxxxxx>
Subject: Re: [Xen-devel] [PATCH] x86: don't write_tsc() non-zero values on CPUs updating only the lower 32 bits
From: Keir Fraser <keir.xen@xxxxxxxxx>
Date: Fri, 15 Apr 2011 08:37:23 +0100
Cc: "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>, "winston.l.wang" <winston.l.wang@xxxxxxxxx>
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Thread-topic: [Xen-devel] [PATCH] x86: don't write_tsc() non-zero values on CPUs updating only the lower 32 bits
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On 15/04/2011 08:08, "Jan Beulich" <JBeulich@xxxxxxxxxx> wrote:

>> But the write is actually tsc + 4*(s32)(tmp-tsc), and tmp has 1U<<32 ORed
>> into it (because it was read after your second write to the TSC. Perhaps we
>> should just write back the full original tsc and call that good enough?
> Again, note the (s32) cast.

Oh yes. Still the 4x is weird, and on this path (!TSC_RELIABLE, TSC is fully
writable) we will sync all AP TSCs as they come up anyway. So writing back
the original TSC value is good enough, as far as this matters at all (which
it probably doesn't).

 -- Keir

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