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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>, 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: "Wang, Winston L" <winston.l.wang@xxxxxxxxx>
Date: Fri, 15 Apr 2011 07:49:41 -0700
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Cc: "Liu, Jinsong" <jinsong.liu@xxxxxxxxx>, Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>, KeirFraser <keir@xxxxxxx>, "Jiang, Yunhong" <yunhong.jiang@xxxxxxxxx>, "Dugger, Donald D" <donald.d.dugger@xxxxxxxxx>, "Li, Xin" <xin.li@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
> -----Original Message-----
> From: Keir Fraser [mailto:keir.xen@xxxxxxxxx]
> Sent: Friday, April 15, 2011 12:37 AM
> 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).
Agree, and new processor use for hot add should be upper 32 bit TSC is 
writeable, I don't think anyone want use those old ones (old model CPU ID 
before family [0FH]) which do not support up32 bit TSC write for hot add. 


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