On Wed, 2023-11-08 at 12:55 -0800, Sean Christopherson wrote: > > Any objection to splitting out the vcpu_info chunk to a separate function? That'd > eliminate the subtle gpc+lock switch, and provide a convenient and noticeable > location to explain why it's ok for setting the vcpu_info to fail. No objection, that looks entirely sane to me. > E.g. > > --- > arch/x86/kvm/xen.c | 102 ++++++++++++++++++++++++++------------------- > 1 file changed, 58 insertions(+), 44 deletions(-) > > diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c > index d65e89e062e4..cd2021ba0ae3 100644 > --- a/arch/x86/kvm/xen.c > +++ b/arch/x86/kvm/xen.c > @@ -1621,6 +1621,57 @@ static void kvm_xen_check_poller(struct kvm_vcpu *vcpu, int port) > } > } > > +static int kvm_xen_needs_a_name(struct kvm_vcpu *vcpu, int port_word_bit) > kvm_xen_deliver_vcpu_evtchn_pending_sel() ? ... > - out_rcu: > + out_unlock: > read_unlock_irqrestore(&gpc->lock, flags); > + > + /* Paul to add comment explaining why it's ok kvm_xen_needs_a_name() fails */ Basically the point here is that it *doesn't* fail. Either it delivers directly to the vcpu_info at the appropriate GPA via the GPC, or it will set the corresponding bit in the 'shadow' vcpu->arch.xen.evtchn_pending_sel to be delivered later by kvm_xen_inject_pending_events(). Either way, it's 'success'. I do want to vet the code path where userspace forgets to reinstate the vcpu_info before running the vCPU again. Looking at kvm_xen_inject_pending_events(), I think the kvm_gpc_check() for the vcpu_info will fail, so it'll just return, and KVM will enter the vCPU with vcpu->arch.xen.evtchn_pending_sel still unchanged? The bits won't get delivered until the next time the vCPU is entered after the vcpu_info is set. Which I *think* is fine. Userspace has to stop running the vCPU in order to actually set the vcpu_info again, when it finally decides to do so. And if it never does, that works out as well as can be expected. Probably does want adding to the selftest.
Attachment:
smime.p7s
Description: S/MIME cryptographic signature
