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RE: [Xen-devel] [PATCH] HVM x86_32 PAE guest support on 64-bit Xen

To: "Keir Fraser" <Keir.Fraser@xxxxxxxxxxxx>
Subject: RE: [Xen-devel] [PATCH] HVM x86_32 PAE guest support on 64-bit Xen
From: "Nakajima, Jun" <jun.nakajima@xxxxxxxxx>
Date: Sat, 18 Feb 2006 18:39:48 -0800
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Thread-topic: [Xen-devel] [PATCH] HVM x86_32 PAE guest support on 64-bit Xen
Keir Fraser wrote:
> On 18 Feb 2006, at 00:26, Nakajima, Jun wrote:
>> The patch enables x86_32 PAE unmodified guests on 64-bit Xen when the
>> hvm feature is present. We tested only Linux at this point, and we'll
>> improve the functionality as we test other guests.
>> The SVM needs the equivalent changes to the vmc.c to get this
>> functionality working, but this patch does not break the build.
> Now that you (wisely) no longer mess with MSR_EFER directly (leaving
> it to the the vmexit/vmentry protocol) can you get rid of the
> VMX_INDEX_MSR_EFER? There's no point in saving/restoring it, since
> it's value will not be different from normal while you run in Xen
> context. 
>   -- Keir

The VMX does not save/restore NXE (bit 11) and SCE (bit 0), and they are
always set in Xen (on machines where VMX is available). So it's possible
that those bits are still effectively on in the guest even after the
guest attempted to clear them (and we ignore such writes). I don't think
bad things can happen practically. We need to check carefully if we
might expose security holes there.

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