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

On 19 Feb 2006, at 07:58, Keir Fraser wrote:

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.

Isn't SYSENTER effectively disabled if SYSENTER_CS == 0? Presumably the guest wouldn't touch that MSR unless it intends to set SCE.

Oops, SCE enables SYSCALL. Yes, that might be a problem -- I think there is no 'safety catch' except for that EFER flag. Possibly the MSR does need save/restore on context switch.

 -- Keir

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