| Message ID | 20240802195120.325560-2-seanjc@google.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | KVM: x86: Fastpath cleanup, fix, and enhancement | expand |
On Fri, Aug 2, 2024 at 9:51 PM Sean Christopherson <seanjc@google.com> wrote: > Re-enter the guest in the fastpath if WRMSR emulation for x2APIC's ICR is > successful, as no additional work is needed, i.e. there is no code unique > for WRMSR exits between the fastpath and the "!= EXIT_FASTPATH_NONE" check > in __vmx_handle_exit(). What about if you send an IPI to yourself? Doesn't that return true for kvm_vcpu_exit_request() if posted interrupts are disabled? Paolo > Signed-off-by: Sean Christopherson <seanjc@google.com> > --- > arch/x86/kvm/x86.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c > index af6c8cf6a37a..cf397110953f 100644 > --- a/arch/x86/kvm/x86.c > +++ b/arch/x86/kvm/x86.c > @@ -2173,7 +2173,7 @@ fastpath_t handle_fastpath_set_msr_irqoff(struct kvm_vcpu *vcpu) > data = kvm_read_edx_eax(vcpu); > if (!handle_fastpath_set_x2apic_icr_irqoff(vcpu, data)) { > kvm_skip_emulated_instruction(vcpu); > - ret = EXIT_FASTPATH_EXIT_HANDLED; > + ret = EXIT_FASTPATH_REENTER_GUEST; > } > break; > case MSR_IA32_TSC_DEADLINE: > -- > 2.46.0.rc2.264.g509ed76dc8-goog >
On Mon, Sep 02, 2024, Paolo Bonzini wrote: > On Fri, Aug 2, 2024 at 9:51 PM Sean Christopherson <seanjc@google.com> wrote: > > Re-enter the guest in the fastpath if WRMSR emulation for x2APIC's ICR is > > successful, as no additional work is needed, i.e. there is no code unique > > for WRMSR exits between the fastpath and the "!= EXIT_FASTPATH_NONE" check > > in __vmx_handle_exit(). > > What about if you send an IPI to yourself? Doesn't that return true > for kvm_vcpu_exit_request() if posted interrupts are disabled? Yes, but that doesn't have anything to do with WRMSR itself, as KVM needs to morph EXIT_FASTPATH_EXIT_HANDLED => EXIT_FASTPATH_REENTER_GUEST if there's a pending event that needs requires injection. Given that kvm_x86_ops.sync_pir_to_irr is likely NULL if virtual interrupt delivery is enabled, the overhead of the trying to re-enter the guest it essentially a few cycles, e.g. check vcpu->mode and kvm_request_pending().
On Tue, Sep 3, 2024 at 5:09 PM Sean Christopherson <seanjc@google.com> wrote: > On Mon, Sep 02, 2024, Paolo Bonzini wrote: > > On Fri, Aug 2, 2024 at 9:51 PM Sean Christopherson <seanjc@google.com> wrote: > > > Re-enter the guest in the fastpath if WRMSR emulation for x2APIC's ICR is > > > successful, as no additional work is needed, i.e. there is no code unique > > > for WRMSR exits between the fastpath and the "!= EXIT_FASTPATH_NONE" check > > > in __vmx_handle_exit(). > > > > What about if you send an IPI to yourself? Doesn't that return true > > for kvm_vcpu_exit_request() if posted interrupts are disabled? > > Yes, but that doesn't have anything to do with WRMSR itself, as KVM needs to morph > EXIT_FASTPATH_EXIT_HANDLED => EXIT_FASTPATH_REENTER_GUEST if there's a pending > event that needs requires injection. The other way round? i.e. treat EXIT_FASTPATH_REENTER_GUEST as EXIT_FASTPATH_EXIT_HANDLED to go through event injection. > Given that kvm_x86_ops.sync_pir_to_irr is likely NULL if virtual interrupt delivery > is enabled, the overhead of the trying to re-enter the guest it essentially a few > cycles, e.g. check vcpu->mode and kvm_request_pending(). No, I wasn't worried about performance. Probably I misread if (likely(exit_fastpath != EXIT_FASTPATH_REENTER_GUEST)) break; as something like if (likely(exit_fastpath == EXIT_FASTPATH_REENTER_GUEST)) continue; EXIT_FASTPATH_REENTER_GUEST is exactly what's needed here. Paolo
On Tue, Sep 03, 2024, Paolo Bonzini wrote: > On Tue, Sep 3, 2024 at 5:09 PM Sean Christopherson <seanjc@google.com> wrote: > > On Mon, Sep 02, 2024, Paolo Bonzini wrote: > > > On Fri, Aug 2, 2024 at 9:51 PM Sean Christopherson <seanjc@google.com> wrote: > > > > Re-enter the guest in the fastpath if WRMSR emulation for x2APIC's ICR is > > > > successful, as no additional work is needed, i.e. there is no code unique > > > > for WRMSR exits between the fastpath and the "!= EXIT_FASTPATH_NONE" check > > > > in __vmx_handle_exit(). > > > > > > What about if you send an IPI to yourself? Doesn't that return true > > > for kvm_vcpu_exit_request() if posted interrupts are disabled? > > > > Yes, but that doesn't have anything to do with WRMSR itself, as KVM needs to morph > > EXIT_FASTPATH_EXIT_HANDLED => EXIT_FASTPATH_REENTER_GUEST if there's a pending > > event that needs requires injection. > > The other way round? i.e. treat EXIT_FASTPATH_REENTER_GUEST as > EXIT_FASTPATH_EXIT_HANDLED to go through event injection. Doh, yes.
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index af6c8cf6a37a..cf397110953f 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -2173,7 +2173,7 @@ fastpath_t handle_fastpath_set_msr_irqoff(struct kvm_vcpu *vcpu) data = kvm_read_edx_eax(vcpu); if (!handle_fastpath_set_x2apic_icr_irqoff(vcpu, data)) { kvm_skip_emulated_instruction(vcpu); - ret = EXIT_FASTPATH_EXIT_HANDLED; + ret = EXIT_FASTPATH_REENTER_GUEST; } break; case MSR_IA32_TSC_DEADLINE:
Re-enter the guest in the fastpath if WRMSR emulation for x2APIC's ICR is successful, as no additional work is needed, i.e. there is no code unique for WRMSR exits between the fastpath and the "!= EXIT_FASTPATH_NONE" check in __vmx_handle_exit(). Signed-off-by: Sean Christopherson <seanjc@google.com> --- arch/x86/kvm/x86.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)