Re: [PATCH] SVM: avoid VMSAVE in ctxt-switch-to

[Jan Beulich <jbeulich@xxxxxxxx>]

On 19.10.2020 16:10, Andrew Cooper wrote:
> On 19/10/2020 14:40, Jan Beulich wrote:
>> Of the state saved by the insn and reloaded by the corresponding VMLOAD
>> - TR, syscall, and sysenter state are invariant while having Xen's state
>>   loaded,
>> - FS, GS, and LDTR are not used by Xen and get suitably set in PV
>>   context switch code.
> 
> I think it would be helpful to state that Xen's state is suitably cached
> in _svm_cpu_up(), as this does now introduce an ordering dependency
> during boot.

I've added a sentence.

> Is it possibly worth putting an assert checking the TR selector?  That
> ought to be good enough to catch stray init reordering problems.

How would checking just the TR selector help? If other pieces of TR
or syscall/sysenter were out of sync, we'd be hosed, too.

I'm also not sure what exactly you mean to do for such an assertion:
Merely check the host VMCB field against TSS_SELECTOR, or do an
actual STR to be closer to what the VMSAVE actually did?

>> Suggested-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> 
> Either way, Acked-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

Thanks.

Jan