linux-kernel - Re: [PATCH v3 01/37] KVM: VMX: Flush all EPTP/VPID contexts on remote TLB flush

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Message-ID: <YQljNBBp/EousNBk@google.com>
Date:   Tue, 3 Aug 2021 15:39:32 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Lai Jiangshan <jiangshanlai+lkml@...il.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 01/37] KVM: VMX: Flush all EPTP/VPID contexts on
 remote TLB flush

On Tue, Aug 03, 2021, Lai Jiangshan wrote:
> (I'm replying to a very old email, so many CCs are dropped.)
> 
> On Sat, Mar 21, 2020 at 5:33 AM Sean Christopherson
> <sean.j.christopherson@...el.com> wrote:
> >
> > Flush all EPTP/VPID contexts if a TLB flush _may_ have been triggered by
> > a remote or deferred TLB flush, i.e. by KVM_REQ_TLB_FLUSH.  Remote TLB
> > flushes require all contexts to be invalidated, not just the active
> > contexts, e.g. all mappings in all contexts for a given HVA need to be
> > invalidated on a mmu_notifier invalidation.  Similarly, the instigator
> > of the deferred TLB flush may be expecting all contexts to be flushed,
> > e.g. vmx_vcpu_load_vmcs().
> >
> > Without nested VMX, flushing only the current EPTP/VPID context isn't
> > problematic because KVM uses a constant VPID for each vCPU, and
> 
> Hello, Sean
> 
> Is the patch optimized for cases where nested VMX is active?

Well, this patch isn't, but KVM has since been optimized to do full EPT/VPID
flushes only when "necessary".  Necessary in quotes because the two uses can
technically be further optimized, but doing so would incur significant complexity.

Use #1 is remote flushes from the MMU, which don't strictly require a global flush,
but KVM would need to propagate more information (mmu_role?) in order for responding
vCPUs to determine what contexts needs to be flushed.  And practically speaking,
for MMU flushes there's no meaningful difference when using TDP without nested
guests as the common case will be that each vCPU has a single active EPTP and
that EPTP will be affected by the MMU changes, i.e. needs to be flushed.

Use #2 is in VMX's pCPU migration path.  Again, not strictly necessary as KVM could
theoretically track which pCPUs have run a particular vCPU and when that pCPU last
flushed EPT contexts, but fully solving the problem would be quite complex.  Since
pCPU migration is always going to be a slow path, the extra complexity would be
very difficult to justify.

> I think the non-nested cases are normal cases.
> 
> Although the related code has been changed, the logic of the patch
> is still working now, would it be better if we restore the optimization
> for the normal cases (non-nested)?

As above, vmx_flush_tlb_all() hasn't changed, but the callers have.