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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 19/10/2020 15:21, Jan Beulich wrote:
> 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.

They're far less likely to move relative to tr, than everything relative
to hvm_up().

> 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?

ASSERT(str() == TSS_SELECTOR);

The problem with the other state is that it compiletime/runtime
dependent, and we don't want to be opencoding the logic a second time.

~Andrew