Re: [Xen-devel] [PATCH 08/15] x86/hvm: Reposition the modification of raw segment data from the VMCB/VMCS

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 11/23/2016 02:28 PM, Andrew Cooper wrote:
>
>> SVM requires attributes of any NULL segment to be zero.
> Where is this claim made?  Vol2 recommends that the VMM clear all
> attributes, but the wording of the previous paragraph suggests that the
> attributes would be ignored in this case.   The %ss bug, and some
> experimentation on my behalf also indicate that they are ignored.

15.5.1 Basic Operation, Segment State in the VMCB:

The VMM should follow these rules when storing segment attributes into
the VMCB
* For NULL segments, set all attribute bits to zero; otherwise, write
the concatenation of bits 55:52 and 47:40 from the original 64-bit
(in-memory) segment descriptors.

I guess the preceding text is indeed unclear as to whether this is
somehow enforced (in which case I am not sure I see the point of having
this rule).

-boris

>
>> I don't know about Intel but if it's the same then should we ASSERT this as 
>> well?
> On Intel if unusable is set, all other bits are ignored.
>
> However, the behaviour of both Intel and AMD is to occasionally set
> upper attribute bits.  At some point I intend to make emulated segment
> loading match d->arch.vendor's behaviour, at which point such an
> ASSERT() would definitely trip.
>
> ~Andrew



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