| Message ID | 20250116035008.43404-1-yosryahmed@google.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | KVM: nVMX: Always use TLB_FLUSH_GUEST for nested VM-Enter/VM-Exit | expand |
On Wed, Jan 15, 2025 at 7:50 PM Yosry Ahmed <yosryahmed@google.com> wrote: > > nested_vmx_transition_tlb_flush() uses KVM_REQ_TLB_FLUSH_CURRENT to > flush the TLB if VPID is enabled for both L1 and L2, but they still > share the TLB tag. This happens if EPT is disabled and KVM fails to > allocate a VPID for L2, so both the EPTP and VPID are shared between L1 > and L2. Nit: Combined and guest-physical TLB tags are based on EPTRTA (the new acronym for EP4TA), not EPTP. But, in any case, with EPT disabled, there are no combined or guest-physical mappings. There are only linear mappings. > Interestingly, nested_vmx_transition_tlb_flush() uses > KVM_REQ_TLB_FLUSH_GUEST to flush the TLB for all other cases where a > flush is required. > > Taking a close look at vmx_flush_tlb_guest() and > vmx_flush_tlb_current(), the main differences are: > (a) vmx_flush_tlb_current() is a noop if the KVM MMU is invalid. > (b) vmx_flush_tlb_current() uses INVEPT if EPT is enabled (instead of > INVVPID) to flush the guest-physical mappings as well as combined > mappings. > > The check in (a) is seemingly an optimization, and there should not be > any TLB entries for L1 anyway if the KVM MMU is invalid. Not having this > check in vmx_flush_tlb_guest() is not a fundamental difference, and it > can be added there separately if needed. > > The difference in (b) is irrelevant in this case, because EPT being > enabled for L1 means that its TLB tags are tagged with EPTP and cannot > be used by L2 (regardless of whether or not EPT is enabled for L2). The difference is also irrelevant because, as you concluded in the first paragraph, EPT is disabled in the final block of nested_vmx_transition_tlb_flush(). > Use KVM_REQ_TLB_FLUSH_GUEST in this case in > nested_vmx_transition_tlb_flush() for consistency. This arguably makes > more sense conceptually too -- L1 and L2 cannot share the TLB tag for > guest-physical translations, so only flushing linear and combined > translations (i.e. guest-generated translations) is needed. And, as I mentioned above, with EPT disabled, there are no combined or guest-physical mappings. > Signed-off-by: Yosry Ahmed <yosryahmed@google.com> I think the reasoning in the commit message can be cleared up a bit, but... Reviewed-by: Jim Mattson <mattson@google.com> > --- > > I tested this by running all selftests that have "nested" in their name > (and not svm). I was tempted to run KVM-unit-tests in an L1 guest but I > convinced myself it's prompted by the change :) > > --- > arch/x86/kvm/vmx/nested.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c > index aa78b6f38dfef..2ed454186e59c 100644 > --- a/arch/x86/kvm/vmx/nested.c > +++ b/arch/x86/kvm/vmx/nested.c > @@ -1241,7 +1241,7 @@ static void nested_vmx_transition_tlb_flush(struct kvm_vcpu *vcpu, > * as the effective ASID is common to both L1 and L2. > */ > if (!nested_has_guest_tlb_tag(vcpu)) > - kvm_make_request(KVM_REQ_TLB_FLUSH_CURRENT, vcpu); > + kvm_make_request(KVM_REQ_TLB_FLUSH_GUEST, vcpu); > } > > static bool is_bitwise_subset(u64 superset, u64 subset, u64 mask) > -- > 2.48.0.rc2.279.g1de40edade-goog > >
On Wed, Jan 15, 2025 at 9:27 PM Jim Mattson <jmattson@google.com> wrote: > > On Wed, Jan 15, 2025 at 7:50 PM Yosry Ahmed <yosryahmed@google.com> wrote: > > > > nested_vmx_transition_tlb_flush() uses KVM_REQ_TLB_FLUSH_CURRENT to > > flush the TLB if VPID is enabled for both L1 and L2, but they still > > share the TLB tag. This happens if EPT is disabled and KVM fails to > > allocate a VPID for L2, so both the EPTP and VPID are shared between L1 > > and L2. > > Nit: Combined and guest-physical TLB tags are based on EPTRTA (the new > acronym for EP4TA), not EPTP. But, in any case, with EPT disabled, > there are no combined or guest-physical mappings. There are only > linear mappings. Interestingly, I did initially write EPTRTA, but I changed it to EPTP because that is the terminology used in nested_has_guest_tlb_tag(). Anyway, I definitely don't mind changing it to EPTRTA. > > > Interestingly, nested_vmx_transition_tlb_flush() uses > > KVM_REQ_TLB_FLUSH_GUEST to flush the TLB for all other cases where a > > flush is required. > > > > Taking a close look at vmx_flush_tlb_guest() and > > vmx_flush_tlb_current(), the main differences are: > > (a) vmx_flush_tlb_current() is a noop if the KVM MMU is invalid. > > (b) vmx_flush_tlb_current() uses INVEPT if EPT is enabled (instead of > > INVVPID) to flush the guest-physical mappings as well as combined > > mappings. > > > > The check in (a) is seemingly an optimization, and there should not be > > any TLB entries for L1 anyway if the KVM MMU is invalid. Not having this > > check in vmx_flush_tlb_guest() is not a fundamental difference, and it > > can be added there separately if needed. > > > > The difference in (b) is irrelevant in this case, because EPT being > > enabled for L1 means that its TLB tags are tagged with EPTP and cannot > > be used by L2 (regardless of whether or not EPT is enabled for L2). > > The difference is also irrelevant because, as you concluded in the > first paragraph, EPT is disabled in the final block of > nested_vmx_transition_tlb_flush(). I was trying to explain that even if EPT is enabled, sharing guest-physical translations between L1 and L2 should never be possible (and hence we should never worry about flushing these translations in nested_vmx_transition_tlb_flush()). Now that I read it again it is not as clear as I had hoped. > > > Use KVM_REQ_TLB_FLUSH_GUEST in this case in > > nested_vmx_transition_tlb_flush() for consistency. This arguably makes > > more sense conceptually too -- L1 and L2 cannot share the TLB tag for > > guest-physical translations, so only flushing linear and combined > > translations (i.e. guest-generated translations) is needed. > > And, as I mentioned above, with EPT disabled, there are no combined or > guest-physical mappings. > > > Signed-off-by: Yosry Ahmed <yosryahmed@google.com> > > I think the reasoning in the commit message can be cleared up a bit, but... Agreed :) I am sure Sean will also want changes in the commit message anyway. > > Reviewed-by: Jim Mattson <mattson@google.com> Thanks for the quick review!
diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c index aa78b6f38dfef..2ed454186e59c 100644 --- a/arch/x86/kvm/vmx/nested.c +++ b/arch/x86/kvm/vmx/nested.c @@ -1241,7 +1241,7 @@ static void nested_vmx_transition_tlb_flush(struct kvm_vcpu *vcpu, * as the effective ASID is common to both L1 and L2. */ if (!nested_has_guest_tlb_tag(vcpu)) - kvm_make_request(KVM_REQ_TLB_FLUSH_CURRENT, vcpu); + kvm_make_request(KVM_REQ_TLB_FLUSH_GUEST, vcpu); } static bool is_bitwise_subset(u64 superset, u64 subset, u64 mask)
nested_vmx_transition_tlb_flush() uses KVM_REQ_TLB_FLUSH_CURRENT to flush the TLB if VPID is enabled for both L1 and L2, but they still share the TLB tag. This happens if EPT is disabled and KVM fails to allocate a VPID for L2, so both the EPTP and VPID are shared between L1 and L2. Interestingly, nested_vmx_transition_tlb_flush() uses KVM_REQ_TLB_FLUSH_GUEST to flush the TLB for all other cases where a flush is required. Taking a close look at vmx_flush_tlb_guest() and vmx_flush_tlb_current(), the main differences are: (a) vmx_flush_tlb_current() is a noop if the KVM MMU is invalid. (b) vmx_flush_tlb_current() uses INVEPT if EPT is enabled (instead of INVVPID) to flush the guest-physical mappings as well as combined mappings. The check in (a) is seemingly an optimization, and there should not be any TLB entries for L1 anyway if the KVM MMU is invalid. Not having this check in vmx_flush_tlb_guest() is not a fundamental difference, and it can be added there separately if needed. The difference in (b) is irrelevant in this case, because EPT being enabled for L1 means that its TLB tags are tagged with EPTP and cannot be used by L2 (regardless of whether or not EPT is enabled for L2). Use KVM_REQ_TLB_FLUSH_GUEST in this case in nested_vmx_transition_tlb_flush() for consistency. This arguably makes more sense conceptually too -- L1 and L2 cannot share the TLB tag for guest-physical translations, so only flushing linear and combined translations (i.e. guest-generated translations) is needed. Signed-off-by: Yosry Ahmed <yosryahmed@google.com> --- I tested this by running all selftests that have "nested" in their name (and not svm). I was tempted to run KVM-unit-tests in an L1 guest but I convinced myself it's prompted by the change :) --- arch/x86/kvm/vmx/nested.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)