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RE: [Xen-devel] [PATCH] small adjustment to asm constraints forc/s19400

To: Jan Beulich <jbeulich@xxxxxxxxxx>, "Lu, Guanqun" <guanqun.lu@xxxxxxxxx>
Subject: RE: [Xen-devel] [PATCH] small adjustment to asm constraints forc/s19400
From: "Tian, Kevin" <kevin.tian@xxxxxxxxx>
Date: Tue, 31 Mar 2009 18:21:19 +0800
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Thread-topic: [Xen-devel] [PATCH] small adjustment to asm constraints forc/s19400
>From: Jan Beulich [mailto:jbeulich@xxxxxxxxxx] 
>Sent: 2009年3月31日 18:15
>>>> "Tian, Kevin" <kevin.tian@xxxxxxxxx> 31.03.09 11:47 >>>
>>Well, got your point. Those three SYSENTER MSRs are lost after resume.
>>We'll verify whether this is the only missing parts.  BTW, in 
>our box 32-on
>>-32 S3 works fine, does that mean SYSENTER/SYSEXIT not used in pure
>>32bit pv environment, since in Xen 32bit path also misses 
>that important
>A 32-bit hypervisor does not expose sysenter to the guest (the 
>feature bit
>gets cleared) - iirc this is to prevent the guest from 
>thinking that using
>sysenter would provide a performance benefit. Hence restoring 
>these MSRs
>in that case would only be needed if Xen itself made use of 
>sysenter, which
>however it doesn't (except for the now only theoretical case of the
>supervisor-mode-kernel feature).

Clear enough. BTW, another question comes when I'm clearing the code.
in load_segments, fs/gs base MSRs are only updated for non 32-on-64
domain. I guess that it's just for performance reason as 32bit guest 
doesn't require to switch gs base. I want to get your confirmation 
since in current low level context S3 resume code, we always restore 
those base MSRs even for 32-on-64 case.

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