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SubjectRe: [PATCH v3 14/37] KVM: x86: Move "flush guest's TLB" logic to separate kvm_x86_ops hook
Date
Sean Christopherson <sean.j.christopherson@intel.com> writes:

> Add a dedicated hook to handle flushing TLB entries on behalf of the
> guest, i.e. for a paravirtualized TLB flush, and use it directly instead
> of bouncing through kvm_vcpu_flush_tlb().
>
> For VMX, change the effective implementation implementation to never do
> INVEPT and flush only the current context, i.e. to always flush via
> INVVPID(SINGLE_CONTEXT). The INVEPT performed by __vmx_flush_tlb() when
> @invalidate_gpa=false and enable_vpid=0 is unnecessary, as it will only
> flush guest-physical mappings; linear and combined mappings are flushed
> by VM-Enter when VPID is disabled, and changes in the guest pages tables
> do not affect guest-physical mappings.
>
> When EPT and VPID are enabled, doing INVVPID is not required (by Intel's
> architecture) to invalidate guest-physical mappings, i.e. TLB entries
> that cache guest-physical mappings can live across INVVPID as the
> mappings are associated with an EPTP, not a VPID. The intent of
> @invalidate_gpa is to inform vmx_flush_tlb() that it must "invalidate
> gpa mappings", i.e. do INVEPT and not simply INVVPID. Other than nested
> VPID handling, which now calls vpid_sync_context() directly, the only
> scenario where KVM can safely do INVVPID instead of INVEPT (when EPT is
> enabled) is if KVM is flushing TLB entries from the guest's perspective,
> i.e. is only required to invalidate linear mappings.
>
> For SVM, flushing TLB entries from the guest's perspective can be done
> by flushing the current ASID, as changes to the guest's page tables are
> associated only with the current ASID.
>
> Adding a dedicated ->tlb_flush_guest() paves the way toward removing
> @invalidate_gpa, which is a potentially dangerous control flag as its
> meaning is not exactly crystal clear, even for those who are familiar
> with the subtleties of what mappings Intel CPUs are/aren't allowed to
> keep across various invalidation scenarios.
>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
> ---
> arch/x86/include/asm/kvm_host.h | 6 ++++++
> arch/x86/kvm/svm.c | 6 ++++++
> arch/x86/kvm/vmx/vmx.c | 13 +++++++++++++
> arch/x86/kvm/x86.c | 2 +-
> 4 files changed, 26 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index cdbf822c5c8b..c08f4c0bf4d1 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -1118,6 +1118,12 @@ struct kvm_x86_ops {
> */
> void (*tlb_flush_gva)(struct kvm_vcpu *vcpu, gva_t addr);
>
> + /*
> + * Flush any TLB entries created by the guest. Like tlb_flush_gva(),
> + * does not need to flush GPA->HPA mappings.
> + */
> + void (*tlb_flush_guest)(struct kvm_vcpu *vcpu);
> +
> void (*run)(struct kvm_vcpu *vcpu);
> int (*handle_exit)(struct kvm_vcpu *vcpu,
> enum exit_fastpath_completion exit_fastpath);
> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> index 08568ae9f7a1..396f42753489 100644
> --- a/arch/x86/kvm/svm.c
> +++ b/arch/x86/kvm/svm.c
> @@ -5643,6 +5643,11 @@ static void svm_flush_tlb_gva(struct kvm_vcpu *vcpu, gva_t gva)
> invlpga(gva, svm->vmcb->control.asid);
> }
>
> +static void svm_flush_tlb_guest(struct kvm_vcpu *vcpu)
> +{
> + svm_flush_tlb(vcpu, false);
> +}
> +
> static void svm_prepare_guest_switch(struct kvm_vcpu *vcpu)
> {
> }
> @@ -7400,6 +7405,7 @@ static struct kvm_x86_ops svm_x86_ops __ro_after_init = {
>
> .tlb_flush = svm_flush_tlb,
> .tlb_flush_gva = svm_flush_tlb_gva,
> + .tlb_flush_guest = svm_flush_tlb_guest,
>
> .run = svm_vcpu_run,
> .handle_exit = handle_exit,
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index ba24bbda2c12..57c1cee58d18 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -2862,6 +2862,18 @@ static void vmx_flush_tlb_gva(struct kvm_vcpu *vcpu, gva_t addr)
> */
> }
>
> +static void vmx_flush_tlb_guest(struct kvm_vcpu *vcpu)
> +{
> + /*
> + * vpid_sync_context() is a nop if vmx->vpid==0, e.g. if enable_vpid==0
> + * or a vpid couldn't be allocated for this vCPU. VM-Enter and VM-Exit
> + * are required to flush GVA->{G,H}PA mappings from the TLB if vpid is
> + * disabled (VM-Enter with vpid enabled and vpid==0 is disallowed),
> + * i.e. no explicit INVVPID is necessary.
> + */
> + vpid_sync_context(to_vmx(vcpu)->vpid);
> +}
> +
> static void vmx_decache_cr0_guest_bits(struct kvm_vcpu *vcpu)
> {
> ulong cr0_guest_owned_bits = vcpu->arch.cr0_guest_owned_bits;
> @@ -7875,6 +7887,7 @@ static struct kvm_x86_ops vmx_x86_ops __ro_after_init = {
>
> .tlb_flush = vmx_flush_tlb,
> .tlb_flush_gva = vmx_flush_tlb_gva,
> + .tlb_flush_guest = vmx_flush_tlb_guest,
>
> .run = vmx_vcpu_run,
> .handle_exit = vmx_handle_exit,
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index f506248d61a1..0b90ec2c93cf 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -2725,7 +2725,7 @@ static void record_steal_time(struct kvm_vcpu *vcpu)
> trace_kvm_pv_tlb_flush(vcpu->vcpu_id,
> st->preempted & KVM_VCPU_FLUSH_TLB);
> if (xchg(&st->preempted, 0) & KVM_VCPU_FLUSH_TLB)
> - kvm_vcpu_flush_tlb(vcpu, false);
> + kvm_x86_ops->tlb_flush_guest(vcpu);
>
> vcpu->arch.st.preempted = 0;

Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>

I *think* I've commented on the previous version that we also have
hyperv-style PV TLB flush and this will likely need to be switched to
tlb_flush_guest(). What do you think about the following (very lightly
tested)?

commit 485b4a579605597b9897b3d9ec118e0f7f1138ad
Author: Vitaly Kuznetsov <vkuznets@redhat.com>
Date: Wed Mar 25 11:14:25 2020 +0100

KVM: x86: make Hyper-V PV TLB flush use tlb_flush_guest()

Hyper-V PV TLB flush mechanism does TLB flush on behalf of the guest
so doing tlb_flush_all() is an overkill, switch to using tlb_flush_guest()
(just like KVM PV TLB flush mechanism) instead. Introduce
KVM_REQ_HV_TLB_FLUSH to support the change.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 167729624149..8c5659ed211b 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -84,6 +84,7 @@
#define KVM_REQ_APICV_UPDATE \
KVM_ARCH_REQ_FLAGS(25, KVM_REQUEST_WAIT | KVM_REQUEST_NO_WAKEUP)
#define KVM_REQ_TLB_FLUSH_CURRENT KVM_ARCH_REQ(26)
+#define KVM_REQ_HV_TLB_FLUSH KVM_ARCH_REQ(27)

#define CR0_RESERVED_BITS \
(~(unsigned long)(X86_CR0_PE | X86_CR0_MP | X86_CR0_EM | X86_CR0_TS \
diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
index a86fda7a1d03..0d051ed11f38 100644
--- a/arch/x86/kvm/hyperv.c
+++ b/arch/x86/kvm/hyperv.c
@@ -1425,8 +1425,7 @@ static u64 kvm_hv_flush_tlb(struct kvm_vcpu *current_vcpu, u64 ingpa,
* vcpu->arch.cr3 may not be up-to-date for running vCPUs so we can't
* analyze it here, flush TLB regardless of the specified address space.
*/
- kvm_make_vcpus_request_mask(kvm,
- KVM_REQ_TLB_FLUSH | KVM_REQUEST_NO_WAKEUP,
+ kvm_make_vcpus_request_mask(kvm, KVM_REQ_HV_TLB_FLUSH,
vcpu_mask, &hv_vcpu->tlb_flush);

ret_success:
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 210af343eebf..5096a9b1a04e 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -2702,6 +2702,12 @@ static void kvm_vcpu_flush_tlb_all(struct kvm_vcpu *vcpu)
kvm_x86_ops->tlb_flush_all(vcpu);
}

+static void kvm_vcpu_flush_tlb_guest(struct kvm_vcpu *vcpu)
+{
+ ++vcpu->stat.tlb_flush;
+ kvm_x86_ops->tlb_flush_guest(vcpu);
+}
+
static void record_steal_time(struct kvm_vcpu *vcpu)
{
struct kvm_host_map map;
@@ -2725,7 +2731,7 @@ static void record_steal_time(struct kvm_vcpu *vcpu)
trace_kvm_pv_tlb_flush(vcpu->vcpu_id,
st->preempted & KVM_VCPU_FLUSH_TLB);
if (xchg(&st->preempted, 0) & KVM_VCPU_FLUSH_TLB)
- kvm_x86_ops->tlb_flush_guest(vcpu);
+ kvm_vcpu_flush_tlb_guest(vcpu);

vcpu->arch.st.preempted = 0;

@@ -8219,7 +8225,8 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
}
if (kvm_check_request(KVM_REQ_TLB_FLUSH_CURRENT, vcpu))
kvm_vcpu_flush_tlb_current(vcpu);
-
+ if (kvm_check_request(KVM_REQ_HV_TLB_FLUSH, vcpu))
+ kvm_vcpu_flush_tlb_guest(vcpu);
if (kvm_check_request(KVM_REQ_REPORT_TPR_ACCESS, vcpu)) {
vcpu->run->exit_reason = KVM_EXIT_TPR_ACCESS;
r = 0;
--
Vitaly

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