Re: [PATCH] KVM: X86: fix tlb_flush_guest()

[Sean Christopherson <seanjc@google.com>]

On Fri, May 28, 2021, Lai Jiangshan wrote:
> 
> On 2021/5/28 00:13, Sean Christopherson wrote:
> > And making a request won't work without revamping the order of request handling
> > in vcpu_enter_guest(), e.g. KVM_REQ_MMU_RELOAD and KVM_REQ_MMU_SYNC are both
> > serviced before KVM_REQ_STEAL_UPDATE.
> 
> Yes, it just fixes the said problem in the simplest way.
> I copied KVM_REQ_MMU_RELOAD from kvm_handle_invpcid(INVPCID_TYPE_ALL_INCL_GLOBAL).
> (If the guest is not preempted, it will call invpcid_flush_all() and will be handled
> by this way)

The problem is that record_steal_time() is called after KVM_REQ_MMU_RELOAD
in vcpu_enter_guest() and so the reload request won't be recognized until the
next VM-Exit.  It works for kvm_handle_invpcid() because vcpu_enter_guest() is
guaranteed to run between the invcpid code and VM-Enter.

> The improvement code will go later, and will not be backported.

I would argue that introducing a potential performance regression is in itself a
bug.  IMO, going straight to kvm_mmu_sync_roots() is not high risk.

> The proper way to flush guest is to use code in
> 
> https://lore.kernel.org/lkml/20210525213920.3340-1-jiangshanlai@gmail.com/
> as:
> +		kvm_mmu_sync_roots(vcpu);
> +		kvm_make_request(KVM_REQ_TLB_FLUSH_CURRENT, vcpu); //or just call flush_current directly
> +		for (i = 0; i < KVM_MMU_NUM_PREV_ROOTS; i++)
> +			vcpu->arch.mmu->prev_roots[i].need_sync = true;
> 
> If need_sync patch is not accepted, we can just use kvm_mmu_sync_roots(vcpu)
> to keep the current pagetable and use kvm_mmu_free_roots() to free all the other
> roots in prev_roots.

I like the idea, I just haven't gotten around to reviewing that patch yet.

> > Cleaning up and documenting the MMU related requests is on my todo list, but the
> > immediate fix should be tiny and I can do my cleanups on top.
> > 
> > I believe the minimal fix is:
> > 
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index 81ab3b8f22e5..b0072063f9bf 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -3072,6 +3072,9 @@ static void kvm_vcpu_flush_tlb_all(struct kvm_vcpu *vcpu)
> >   static void kvm_vcpu_flush_tlb_guest(struct kvm_vcpu *vcpu)
> >   {
> >          ++vcpu->stat.tlb_flush;
> > +
> > +       if (!tdp_enabled)
> > +               kvm_mmu_sync_roots(vcpu);
> 
> it doesn't handle prev_roots which are also needed as
> shown in kvm_handle_invpcid(INVPCID_TYPE_ALL_INCL_GLOBAL).

Ya, I belated realized this :-)

> >          static_call(kvm_x86_tlb_flush_guest)(vcpu);
> 
> For tdp_enabled, I think it is better to use kvm_x86_tlb_flush_current()
> to make it consistent with other shadowpage code.
> 
> >   }
> >