Re: [Xen-devel] [PATCH 1/2] Revert "x86/hvm: disable pkeys for guests in non-paging mode"

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 26.05.17 at 19:03, <andrew.cooper3@xxxxxxxxxx> wrote:
> This reverts commit c41e0266dd59ab50b7a153157e9bd2a3ad114b53.
> 
> When determining Access Rights, Protection Keys only take effect when CR4.PKE
> it set, and 4-level paging is active.  All other circumstances (notibly, 32bit
> PAE paging) skip the Protection Key control mechanism.
> 
> Therefore, we do not need to clear CR4.PKE behind the back of a guest which is
> not using paging, as such a guest is necesserily running with EFER.LME
> disabled.

DYM EFER.LMA here?

> The {RD,WR}PKRU instructions are specified as being legal for use in any
> operating mode, but only if CR4.PKE is set.  By clearing CR4.PKE behind the
> back of an unpaged guest, these instructions yield #UD despite the guest
> seeing PKE set if it reads CR4, and OSPKE being visible in CPUID.
> 
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>

I would like to get clarification from Huaitong, however, on the
reason for the original change: According to the reasoning here,
it shouldn't have been an observed failure of some kind, but
merely the thinking that something may be wrong (but really
wasn't).

Jan


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