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Re: [Xen-devel] [PATCH 0/4] XSAVE/XRSTOR fixes and enhancements

To: "Han, Weidong" <weidong.han@xxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>
Subject: Re: [Xen-devel] [PATCH 0/4] XSAVE/XRSTOR fixes and enhancements
From: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
Date: Tue, 31 Aug 2010 07:52:57 +0100
Cc: Tim Deegan <Tim.Deegan@xxxxxxxxxxxxx>, Jeremy Fitzhardinge <jeremy@xxxxxxxx>
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Thread-topic: [Xen-devel] [PATCH 0/4] XSAVE/XRSTOR fixes and enhancements
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On 31/08/2010 03:59, "Han, Weidong" <weidong.han@xxxxxxxxx> wrote:

> The patchset is as follows:
> Patch 1/4: Fix frozen states
> If a guest sets a state and dirties the state, but later temporarily clears
> the state, and at this time this vcpu is scheduled out, then other vcpus may
> corrupt the state before the vcpu is scheduled in again, thus the state cannot
> be restored correctly. To solve this issue, this patch save/restore all states
> unconditionally on vcpu context switch.

I don't really understand, and the patch is non-obvious and large. Can it be
broken up or explained more.

> Patch 2/4: Enable XSAVE/XRSTOR live migration:
> This patch enables save/restore xsave states for guests. Because xsave area
> size is not fixed, this patch uses 4K Bytes for future extension.

Breaks backward compatibility by changing size of vcpu_guest_context (part
of the PV guest ABI). Totally unacceptable -- is this a nasty attempt to get
SR working for PV guests? I think you'll be needing a special XSAVE
save/restore domctl for PV. However I think you're targeting HVM only for
now so I'm not sure why this is at all needed.

Also expanding the size of the hvm_vcpu s/r structure similarly breaks
compatibility with older saved images. You'll have to put XSAVE state in a
separate s/r chunk with its own identifier tag. I'm not sure we currently
support variable-length HVM s/r chunks but there's no reason not to and this
XSAVE chunk would be a good time to implement that. Then you don't need to
limit yourself to a static 4kB for all time.

 -- Keir

> Patch 3/4. Enable guest AVX
> This patch enables Intel(R) Advanced Vector Extension (AVX) for guest.
> Patch 4/4. Dom0: extend fpu_ctxt to consist with Xen hypervisor
> fpu_ctxt must be consistent between Dom0 and Xen hypervisor. Otherwise, dom0
> cannot boot up.
> Signed-off-by: Weidong Han <weidong.han@xxxxxxxxx>

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