2022-10-01 01:37:27

by Sean Christopherson

Subject: [PATCH v4 09/32] KVM: x86: Handle APICv updates for APIC "mode" changes via request

Use KVM_REQ_UPDATE_APICV to react to APIC "mode" changes, i.e. to handle
the APIC being hardware enabled/disabled and/or x2APIC being toggled.
There is no need to immediately update APICv state, the only requirement
is that APICv be updating prior to the next VM-Enter.

Making a request will allow piggybacking KVM_REQ_UPDATE_APICV to "inhibit"
the APICv memslot when x2APIC is enabled. Doing that directly from
kvm_lapic_set_base() isn't feasible as KVM's SRCU must not be held when
modifying memslots (to avoid deadlock), and may or may not be held when
kvm_lapic_set_base() is called, i.e. KVM can't do the right thing without
tracking that is rightly buried behind CONFIG_PROVE_RCU=y.

Suggested-by: Maxim Levitsky <[email protected]>
Signed-off-by: Sean Christopherson <[email protected]>
---
arch/x86/kvm/lapic.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index 2503f162eb95..316b61b56cca 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -2401,7 +2401,7 @@ void kvm_lapic_set_base(struct kvm_vcpu *vcpu, u64 value)
kvm_apic_set_x2apic_id(apic, vcpu->vcpu_id);

if ((old_value ^ value) & (MSR_IA32_APICBASE_ENABLE | X2APIC_ENABLE)) {
- kvm_vcpu_update_apicv(vcpu);
+ kvm_make_request(KVM_REQ_APICV_UPDATE, vcpu);
static_call_cond(kvm_x86_set_virtual_apic_mode)(vcpu);
}

--
2.38.0.rc1.362.ged0d419d3c-goog

2022-12-08 22:38:39

by Maxim Levitsky

Subject: Re: [PATCH v4 09/32] KVM: x86: Handle APICv updates for APIC "mode" changes via request

On Sat, 2022-10-01 at 00:58 +0000, Sean Christopherson wrote:
> Use KVM_REQ_UPDATE_APICV to react to APIC "mode" changes, i.e. to handle
> the APIC being hardware enabled/disabled and/or x2APIC being toggled.
> There is no need to immediately update APICv state, the only requirement
> is that APICv be updating prior to the next VM-Enter.
>
> Making a request will allow piggybacking KVM_REQ_UPDATE_APICV to "inhibit"
> the APICv memslot when x2APIC is enabled. Doing that directly from
> kvm_lapic_set_base() isn't feasible as KVM's SRCU must not be held when
> modifying memslots (to avoid deadlock), and may or may not be held when
> kvm_lapic_set_base() is called, i.e. KVM can't do the right thing without
> tracking that is rightly buried behind CONFIG_PROVE_RCU=y.
>
> Suggested-by: Maxim Levitsky <[email protected]>
> Signed-off-by: Sean Christopherson <[email protected]>
> ---
> arch/x86/kvm/lapic.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index 2503f162eb95..316b61b56cca 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -2401,7 +2401,7 @@ void kvm_lapic_set_base(struct kvm_vcpu *vcpu, u64 value)
> kvm_apic_set_x2apic_id(apic, vcpu->vcpu_id);
>
> if ((old_value ^ value) & (MSR_IA32_APICBASE_ENABLE | X2APIC_ENABLE)) {
> - kvm_vcpu_update_apicv(vcpu);
> + kvm_make_request(KVM_REQ_APICV_UPDATE, vcpu);
> static_call_cond(kvm_x86_set_virtual_apic_mode)(vcpu);
> }
>

Makes sense.

Reviewed-by: Maxim Levitsky <[email protected]>

Best regards,
Maxim Levitsky