Re: [PATCH v8 05/12] x86emul: support X{SUS,RES}LDTRK

[Jan Beulich <jbeulich@xxxxxxxx>]

On 08.05.2020 15:15, Andrew Cooper wrote:
> On 08/05/2020 08:38, Jan Beulich wrote:
>> [CAUTION - EXTERNAL EMAIL] DO NOT reply, click links, or open attachments 
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>>
>> On 07.05.2020 22:13, Andrew Cooper wrote:
>>> On 05/05/2020 09:14, Jan Beulich wrote:
>>>> --- a/xen/tools/gen-cpuid.py
>>>> +++ b/xen/tools/gen-cpuid.py
>>>> @@ -284,6 +284,9 @@ def crunch_numbers(state):
>>>>          # as dependent features simplifies Xen's logic, and prevents the 
>>>> guest
>>>>          # from seeing implausible configurations.
>>>>          IBRSB: [STIBP, SSBD],
>>>> +
>>>> +        # In principle the TSXLDTRK insns could also be considered 
>>>> independent.
>>>> +        RTM: [TSXLDTRK],
>>> Why the link?  There is no relevant interaction AFAICT.
>> Do the insns make any sense without TSX? Anyway - hence the
>> comment, and if you're convinced the connection does not
>> need making, I'd be okay dropping it. I would assume though
>> that we'd better hide TSXLDTRK whenever we hide RTM, which
>> is most easily achieved by having a connection here.
> 
> Actually - that is a very good point.  I expect there will (or should)
> be an interaction with MSR_TSX_CTRL, as it has CPUID-hiding functionality.
> 
> For now, could I ask you to not expose this to guests in this patch?

As per our irc discussion, I'd make it 'a' then instead of 'A'.
Will need to wait for gen-cpuid.py to accept 'a' then.

> For the emulator side of things alone I think this is ok (although
> looking over it a second time, we could really do with a comment in the
> code explaining that we're never in an RTM region, hence the nop behaviour).

I've added

                /*
                 * We're never in a transactional region when coming here
                 * - nothing else to do.
                 */

to both paths.

> I'll follow up with Intel, and we can figure out the CPUID derivation
> details at a later point.
> 
> If you're happy with this plan, then A-by to save a round trip.

Thanks.

Jan