Re: [PATCH v3] x86/speculation, KVM: only IBPB for switch_mm_always_ibpb on vCPU load

Jon Kohler
2022-05-06T15:43:25Z

> On Apr 30, 2022, at 12:08 PM, Borislav Petkov <bp@alien8.de> wrote:
>
> On Sat, Apr 30, 2022 at 02:50:35PM +0000, Jon Kohler wrote:
>> This is 100% a fair ask, I appreciate the diligence, as we’ve all been there
>> on the ‘other side’ of changes to complex areas and spend hours digging on
>> git history, LKML threads, SDM/APM, and other sources trying to derive
>> why the heck something is the way it is.
>
> Yap, that's basically proving my point and why I want stuff to be
> properly documented so that the question "why was it done this way" can
> always be answered satisfactorily.
>
>> AFAIK, the KVM IBPB is avoided when switching in between vCPUs
>> belonging to the same vmcs/vmcb (i.e. the same guest), e.g. you could
>> have one VM highly oversubscribed to the host and you wouldn’t see
>> either the KVM IBPB or the switch_mm IBPB. All good.
>>
>> Reference vmx_vcpu_load_vmcs() and svm_vcpu_load() and the
>> conditionals prior to the barrier.
>
> So this is where something's still missing.
>
>> However, the pain ramps up when you have a bunch of separate guests,
>> especially with a small amount of vCPUs per guest, so the switching is more
>> likely to be in between completely separate guests.
>
> If the guests are completely separate, then it should fall into the
> switch_mm() case.
>
> Unless it has something to do with, as I looked at the SVM side of
> things, the VMCBs:
>
> if (sd->current_vmcb != svm->vmcb) {
>
> So it is not only different guests but also within the same guest and
> when the VMCB of the vCPU is not the current one.
>
> But then if VMCB of the vCPU is not the current, per-CPU VMCB, then that
> CPU ran another guest so in order for that other guest to attack the
> current guest, then its branch pred should be flushed.
>
> But I'm likely missing a virt aspect here so I'd let Sean explain what
> the rules are...

Hey Sean - touching back on this one to see if were able to get some
thoughts together on this one?

Thanks - Jon

>
> Thx.
>
> --
> Regards/Gruss,
> Boris.
>
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