Re: [PATCH 1/5] KVM: x86: Re-enter guest if WRMSR(X2APIC_ICR) fastpath is successful

[Paolo Bonzini]

On Tue, Sep 3, 2024 at 5:09 PM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
> On Mon, Sep 02, 2024, Paolo Bonzini wrote:
> > On Fri, Aug 2, 2024 at 9:51 PM Sean Christopherson <seanjc@xxxxxxxxxx> 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