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SubjectRe: [PATCH v2 05/10] KVM: VMX: Invalidate hv_tlb_eptp to denote an EPTP mismatch
On Wed, Oct 21, 2020 at 02:39:20PM +0200, Vitaly Kuznetsov wrote:
> Sean Christopherson <sean.j.christopherson@intel.com> writes:
>
> > Drop the dedicated 'ept_pointers_match' field in favor of stuffing
> > 'hv_tlb_eptp' with INVALID_PAGE to mark it as invalid, i.e. to denote
> > that there is at least one EPTP mismatch. Use a local variable to
> > track whether or not a mismatch is detected so that hv_tlb_eptp can be
> > used to skip redundant flushes.
> >
> > No functional change intended.
> >
> > Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
> > ---
> > arch/x86/kvm/vmx/vmx.c | 16 ++++++++--------
> > arch/x86/kvm/vmx/vmx.h | 7 -------
> > 2 files changed, 8 insertions(+), 15 deletions(-)
> >
> > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> > index 52cb9eec1db3..4dfde8b64750 100644
> > --- a/arch/x86/kvm/vmx/vmx.c
> > +++ b/arch/x86/kvm/vmx/vmx.c
> > @@ -498,13 +498,13 @@ static int hv_remote_flush_tlb_with_range(struct kvm *kvm,
> > struct kvm_vmx *kvm_vmx = to_kvm_vmx(kvm);
> > struct kvm_vcpu *vcpu;
> > int ret = 0, i;
> > + bool mismatch;
> > u64 tmp_eptp;
> >
> > spin_lock(&kvm_vmx->ept_pointer_lock);
> >
> > - if (kvm_vmx->ept_pointers_match != EPT_POINTERS_MATCH) {
> > - kvm_vmx->ept_pointers_match = EPT_POINTERS_MATCH;
> > - kvm_vmx->hv_tlb_eptp = INVALID_PAGE;
> > + if (!VALID_PAGE(kvm_vmx->hv_tlb_eptp)) {
> > + mismatch = false;
> >
> > kvm_for_each_vcpu(i, vcpu, kvm) {
> > tmp_eptp = to_vmx(vcpu)->ept_pointer;
> > @@ -515,12 +515,13 @@ static int hv_remote_flush_tlb_with_range(struct kvm *kvm,
> > if (!VALID_PAGE(kvm_vmx->hv_tlb_eptp))
> > kvm_vmx->hv_tlb_eptp = tmp_eptp;
> > else
> > - kvm_vmx->ept_pointers_match
> > - = EPT_POINTERS_MISMATCH;
> > + mismatch = true;
> >
> > ret |= hv_remote_flush_eptp(tmp_eptp, range);
> > }
> > - } else if (VALID_PAGE(kvm_vmx->hv_tlb_eptp)) {
> > + if (mismatch)
> > + kvm_vmx->hv_tlb_eptp = INVALID_PAGE;
> > + } else {
> > ret = hv_remote_flush_eptp(kvm_vmx->hv_tlb_eptp, range);
> > }
>
> Personally, I find double negations like 'mismatch = false' hard to read
> :-).

Paolo also dislikes double negatives (I just wasted a minute of my life trying
to work a double negative into that sentence).

> What if we write this all like
>
> if (!VALID_PAGE(kvm_vmx->hv_tlb_eptp)) {
> kvm_vmx->hv_tlb_eptp = to_vmx(vcpu0)->ept_pointer;
> kvm_for_each_vcpu() {
> tmp_eptp = to_vmx(vcpu)->ept_pointer;
> if (!VALID_PAGE(tmp_eptp) || tmp_eptp != kvm_vmx->hv_tlb_eptp)
> kvm_vmx->hv_tlb_eptp = INVALID_PAGE;
> if (VALID_PAGE(tmp_eptp))
> ret |= hv_remote_flush_eptp(tmp_eptp, range);
> }
> } else {
> ret = hv_remote_flush_eptp(kvm_vmx->hv_tlb_eptp, range);
> }
>
> (not tested and I've probably missed something)

It works, but doesn't optimize the case where one or more vCPUs has an invalid
EPTP. E.g. if vcpuN->ept_pointer is INVALID_PAGE, vcpuN+1..vcpuZ will flush,
even if they all match. Now, whether or not it's worth optimizing that case...

This is also why I named it "mismatch", i.e. it tracks whether or not there was
a mismatch between valid EPTPs, not that all EPTPs matched.

What about replacing "mismatch" with a counter that tracks the number of unique,
valid PGDs that are encountered?

if (!VALID_PAGE(kvm_vmx->hv_tlb_pgd)) {
unique_valid_pgd_cnt = 0;

kvm_for_each_vcpu(i, vcpu, kvm) {
tmp_pgd = to_vmx(vcpu)->hv_tlb_pgd;
if (!VALID_PAGE(tmp_pgd) ||
tmp_pgd == kvm_vmx->hv_tlb_pgd)
continue;

unique_valid_pgd_cnt++;

if (!VALID_PAGE(kvm_vmx->hv_tlb_pgd))
kvm_vmx->hv_tlb_pgd = tmp_pgd;

if (!ret)
ret = hv_remote_flush_pgd(tmp_pgd, range);

if (ret && unique_valid_pgd_cnt > 1)
break;
}
if (unique_valid_pgd_cnt > 1)
kvm_vmx->hv_tlb_pgd = INVALID_PAGE;
} else {
ret = hv_remote_flush_pgd(kvm_vmx->hv_tlb_pgd, range);
}


Alternatively, the pgd_cnt adjustment could be used to update hv_tlb_pgd, e.g.

if (++unique_valid_pgd_cnt == 1)
kvm_vmx->hv_tlb_pgd = tmp_pgd;

I think I like this last one the most. It self-documents what we're tracking
as well as the relationship between the number of valid PGDs and hv_tlb_pgd.

I'll also add a few comments to explain how kvm_vmx->hv_tlb_pgd is used.

Thoughts?

> > @@ -3042,8 +3043,7 @@ static void vmx_load_mmu_pgd(struct kvm_vcpu *vcpu, unsigned long pgd,
> > if (kvm_x86_ops.tlb_remote_flush) {
> > spin_lock(&to_kvm_vmx(kvm)->ept_pointer_lock);
> > to_vmx(vcpu)->ept_pointer = eptp;
> > - to_kvm_vmx(kvm)->ept_pointers_match
> > - = EPT_POINTERS_CHECK;
> > + to_kvm_vmx(kvm)->hv_tlb_eptp = INVALID_PAGE;
> > spin_unlock(&to_kvm_vmx(kvm)->ept_pointer_lock);
> > }
> >
> > diff --git a/arch/x86/kvm/vmx/vmx.h b/arch/x86/kvm/vmx/vmx.h
> > index 3d557a065c01..e8d7d07b2020 100644
> > --- a/arch/x86/kvm/vmx/vmx.h
> > +++ b/arch/x86/kvm/vmx/vmx.h
> > @@ -288,12 +288,6 @@ struct vcpu_vmx {
> > } shadow_msr_intercept;
> > };
> >
> > -enum ept_pointers_status {
> > - EPT_POINTERS_CHECK = 0,
> > - EPT_POINTERS_MATCH = 1,
> > - EPT_POINTERS_MISMATCH = 2
> > -};
> > -
> > struct kvm_vmx {
> > struct kvm kvm;
> >
> > @@ -302,7 +296,6 @@ struct kvm_vmx {
> > gpa_t ept_identity_map_addr;
> >
> > hpa_t hv_tlb_eptp;
> > - enum ept_pointers_status ept_pointers_match;
> > spinlock_t ept_pointer_lock;
> > };
>
> --
> Vitaly
>

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