Re: [PATCH v3 07/28] KVM: x86: Inhibit APIC memslot if x2APIC and AVIC are enabled

Maxim Levitsky
2022-09-23T10:28:09Z

On Tue, 2022-09-20 at 23:31 +0000, Sean Christopherson wrote:
> Free the APIC access page memslot if any vCPU enables x2APIC and SVM's
> AVIC is enabled to prevent accesses to the virtual APIC on vCPUs with
> x2APIC enabled. On AMD, due to its "hybrid" mode where AVIC is enabled
> when x2APIC is enabled even without x2AVIC support, keeping the APIC
> access page memslot results in the guest being able to access the virtual
> APIC page as x2APIC is fully emulated by KVM. I.e. hardware isn't aware
> that the guest is operating in x2APIC mode.
>
> Intel doesn't suffer from the same issue as APICv has fully independent
> VMCS controls for xAPIC vs. x2APIC virtualization. Technically, KVM
> should provide bus error semantics and not memory semantics for the APIC
> page when x2APIC is enabled, but KVM already provides memory semantics in
> other scenarios, e.g. if APICv/AVIC is enabled and the APIC is hardware
> disabled (via APIC_BASE MSR).
>
> Reserve an inhibit bit so that common code can detect whether or not the
> "x2APIC inhibit" applies, but use a dedicated flag to track the inhibit
> so that it doesn't need to be stripped from apicv_inhibit_reasons (since
> it's not a "full" inhibit).
>
> Note, setting apic_access_memslot_inhibited without taking locks relies
> on it being sticky, and also relies on apic_access_memslot_enabled being
> set during vCPU creation (before kvm_vcpu_reset()). vCPUs can race to
> set the inhibit and delete the memslot, i.e. can get false positives, but
> can't false negatives as apic_access_memslot_enabled can't be toggle "on"
> once any vCPU reaches kvm_lapic_set_base().
>
> Opportunistically drop the "can" while updating avic_activate_vmcb()'s
> comment, i.e. to state that KVM _does_ support the hybrid mode. Move
> the "Note:" down a line to conform to preferred kernel/KVM multi-line
> comment style.
>
> Opportunistically update the apicv_update_lock comment, as it isn't
> actually used to protect apic_access_memslot_enabled (it's protected by
> slots_lock).
>
> Fixes: 0e311d33bfbe ("KVM: SVM: Introduce hybrid-AVIC mode")
> Cc: stable@vger.kernel.org
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
> arch/x86/include/asm/kvm_host.h | 18 ++++++++++++++---
> arch/x86/kvm/lapic.c | 34 +++++++++++++++++++++++++++++++--
> arch/x86/kvm/lapic.h | 1 +
> arch/x86/kvm/svm/avic.c | 15 ++++++++-------
> arch/x86/kvm/x86.c | 7 +++++++
> 5 files changed, 63 insertions(+), 12 deletions(-)
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 2c96c43c313a..6475c882b359 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -1132,6 +1132,17 @@ enum kvm_apicv_inhibit {
> * AVIC is disabled because SEV doesn't support it.
> */
> APICV_INHIBIT_REASON_SEV,
> +
> + /*
> + * Due to sharing page tables across vCPUs, the xAPIC memslot must be
> + * deleted if any vCPU has x2APIC enabled as SVM doesn't provide fully
> + * independent controls for AVIC vs. x2AVIC, and also because SVM
> + * supports a "hybrid" AVIC mode for CPUs that support AVIC but not
> + * x2AVIC. Note, this isn't a "full" inhibit and is tracked separately.
> + * AVIC can still be activated, but KVM must not create SPTEs for the
> + * APIC base. For simplicity, this is sticky.
> + */
> + APICV_INHIBIT_REASON_X2APIC,

Hi Sean!

So assuming that I won't object to making it SVM specific (I still think
that VMX should also inhibit this memslot because this is closer to x86 spec,
but if you really want it this way, I won't fight over it):

I somewhat don't like this inhibit, because now it is used just to say
'I am AVIC'.

What do you think if you just move the code that removes the memslot to SVM,
to avic_set_virtual_apic_mode?

> };
>
> struct kvm_arch {
> @@ -1169,10 +1180,11 @@ struct kvm_arch {
> struct kvm_apic_map __rcu *apic_map;
> atomic_t apic_map_dirty;
>
> - /* Protects apic_access_memslot_enabled and apicv_inhibit_reasons */
> - struct rw_semaphore apicv_update_lock;
> -
> bool apic_access_memslot_enabled;
> + bool apic_access_memslot_inhibited;

So the apic_access_memslot_enabled currently tracks if the memslot is enabled.
As I see later in the patch when you free the memslot, you set it to false,
which means that if a vCPU is created after that (it can happen in theory),
the memslot will be created again :(

I say we need 'enabled', and 'allocated' booleans instead. Inhibit will set
enabled to false, and then on next vcpu run, that will free the memslot.

when enabled == false, the code needs to be changed to not allocate it again.

> +
> + /* Protects apicv_inhibit_reasons */
> + struct rw_semaphore apicv_update_lock;
> unsigned long apicv_inhibit_reasons;
>
> gpa_t wall_clock;
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index 99994d2470a2..70f00eda75b2 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -2394,9 +2394,26 @@ void kvm_lapic_set_base(struct kvm_vcpu *vcpu, u64 value)
> }
> }
>
> - if (((old_value ^ value) & X2APIC_ENABLE) && (value & X2APIC_ENABLE))
> + if (((old_value ^ value) & X2APIC_ENABLE) && (value & X2APIC_ENABLE)) {
> kvm_apic_set_x2apic_id(apic, vcpu->vcpu_id);
>
> + /*
> + * Mark the APIC memslot as inhibited if x2APIC is enabled and
> + * the x2APIC inhibit is required. The actual deletion of the
> + * memslot is handled by vcpu_run() as SRCU may or may not be
> + * held at this time, i.e. updating memslots isn't safe. Don't
> + * check apic_access_memslot_inhibited, this vCPU needs to
> + * ensure the memslot is deleted before re-entering the guest,
> + * i.e. needs to make the request even if the inhibit flag was
> + * already set by a different vCPU.
> + */
> + if (vcpu->kvm->arch.apic_access_memslot_enabled &&
> + static_call(kvm_x86_check_apicv_inhibit_reasons)(APICV_INHIBIT_REASON_X2APIC)) {
> + vcpu->kvm->arch.apic_access_memslot_inhibited = true;
> + kvm_make_request(KVM_REQ_UNBLOCK, vcpu);

You are about to remove the KVM_REQ_UNBLOCK in other patch series.

How about just raising KVM_REQ_APICV_UPDATE on current vCPU
and having a special case in kvm_vcpu_update_apicv of

if (apic_access_memslot_enabled == false && apic_access_memslot_allocaed == true) {
drop srcu lock
free the memslot
take srcu lock
}

That wasn't possible to do with regular AVIC inhibit as I tried, because it has to be done
before any vCPU re-enters the guest, so the KVM_REQ_APICV_UPDATE has to be raised
on all vCPUs, and then a 'winning' vCPU would toggle the memslot, but that
would cause a lot of impossible to solve races, thus I abandoned that approach
back then.

Here the accuracy is not that critical so we can raise the request on current vCPU,
as you do in your patch anyway, so it should work.

> + }
> + }
> +
> if ((old_value ^ value) & (MSR_IA32_APICBASE_ENABLE | X2APIC_ENABLE)) {
> kvm_vcpu_update_apicv(vcpu);
> static_call_cond(kvm_x86_set_virtual_apic_mode)(vcpu);
> @@ -2440,7 +2457,8 @@ int kvm_alloc_apic_access_page(struct kvm *kvm)
> int ret = 0;
>
> mutex_lock(&kvm->slots_lock);
> - if (kvm->arch.apic_access_memslot_enabled)
> + if (kvm->arch.apic_access_memslot_enabled ||
> + kvm->arch.apic_access_memslot_inhibited)
> goto out;
>
> hva = __x86_set_memory_region(kvm, APIC_ACCESS_PAGE_PRIVATE_MEMSLOT,
> @@ -2468,6 +2486,18 @@ int kvm_alloc_apic_access_page(struct kvm *kvm)
> }
> EXPORT_SYMBOL_GPL(kvm_alloc_apic_access_page);
>
> +void kvm_free_apic_access_page(struct kvm *kvm)
> +{
> + mutex_lock(&kvm->slots_lock);
> +
> + if (kvm->arch.apic_access_memslot_enabled) {
> + __x86_set_memory_region(kvm, APIC_ACCESS_PAGE_PRIVATE_MEMSLOT, 0, 0);
> + kvm->arch.apic_access_memslot_enabled = false;
> + }
> +
> + mutex_unlock(&kvm->slots_lock);
> +}
> +
> void kvm_lapic_reset(struct kvm_vcpu *vcpu, bool init_event)
> {
> struct kvm_lapic *apic = vcpu->arch.apic;
> diff --git a/arch/x86/kvm/lapic.h b/arch/x86/kvm/lapic.h
> index 6d06397683d0..e2271ffa7ac0 100644
> --- a/arch/x86/kvm/lapic.h
> +++ b/arch/x86/kvm/lapic.h
> @@ -112,6 +112,7 @@ int kvm_apic_set_irq(struct kvm_vcpu *vcpu, struct kvm_lapic_irq *irq,
> int kvm_apic_local_deliver(struct kvm_lapic *apic, int lvt_type);
> void kvm_apic_update_apicv(struct kvm_vcpu *vcpu);
> int kvm_alloc_apic_access_page(struct kvm *kvm);
> +void kvm_free_apic_access_page(struct kvm *kvm);
>
> bool kvm_irq_delivery_to_apic_fast(struct kvm *kvm, struct kvm_lapic *src,
> struct kvm_lapic_irq *irq, int *r, struct dest_map *dest_map);
> diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
> index 0424a5e664bb..8f9426f21bbf 100644
> --- a/arch/x86/kvm/svm/avic.c
> +++ b/arch/x86/kvm/svm/avic.c
> @@ -72,12 +72,12 @@ static void avic_activate_vmcb(struct vcpu_svm *svm)
>
> vmcb->control.int_ctl |= AVIC_ENABLE_MASK;
>
> - /* Note:
> - * KVM can support hybrid-AVIC mode, where KVM emulates x2APIC
> - * MSR accesses, while interrupt injection to a running vCPU
> - * can be achieved using AVIC doorbell. The AVIC hardware still
> - * accelerate MMIO accesses, but this does not cause any harm
> - * as the guest is not supposed to access xAPIC mmio when uses x2APIC.
> + /*
> + * Note: KVM supports hybrid-AVIC mode, where KVM emulates x2APIC MSR
> + * accesses, while interrupt injection to a running vCPU can be
> + * achieved using AVIC doorbell. KVM disables the APIC access page
> + * (deletes the memslot) if any vCPU has x2APIC enabled, thus enabling
> + * AVIC in hybrid mode activates only the doorbell mechanism.
> */
> if (apic_x2apic_mode(svm->vcpu.arch.apic) &&
> avic_mode == AVIC_MODE_X2) {
> @@ -987,7 +987,8 @@ bool avic_check_apicv_inhibit_reasons(enum kvm_apicv_inhibit reason)
> BIT(APICV_INHIBIT_REASON_BLOCKIRQ) |
> BIT(APICV_INHIBIT_REASON_SEV) |
> BIT(APICV_INHIBIT_REASON_APIC_ID_MODIFIED) |
> - BIT(APICV_INHIBIT_REASON_APIC_BASE_MODIFIED);
> + BIT(APICV_INHIBIT_REASON_APIC_BASE_MODIFIED) |
> + BIT(APICV_INHIBIT_REASON_X2APIC);
>
> return supported & BIT(reason);
> }
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index d7374d768296..aa5ab0c620de 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -10705,6 +10705,13 @@ static int vcpu_run(struct kvm_vcpu *vcpu)
> break;
> }
>
> + if (vcpu->kvm->arch.apic_access_memslot_inhibited &&
> + vcpu->kvm->arch.apic_access_memslot_enabled) {
> + kvm_vcpu_srcu_read_unlock(vcpu);
> + kvm_free_apic_access_page(vcpu->kvm);
> + kvm_vcpu_srcu_read_lock(vcpu);
> + }
> +
> if (__xfer_to_guest_mode_work_pending()) {
> kvm_vcpu_srcu_read_unlock(vcpu);
> r = xfer_to_guest_mode_handle_work(vcpu);

I will review the rest of the patch series on next Wednesday (after
our holidays).
From a quick glance they look good to me.

Thanks,
Best regards,
Maxim Levitsky