Re: [PATCH v2 4/7] kvm/mips: rework guest entry logic

[Paolo Bonzini]

On 1/20/22 17:44, Mark Rutland wrote:
> On Wed, Jan 19, 2022 at 10:58:51AM +0000, Mark Rutland wrote:
> > In kvm_arch_vcpu_ioctl_run() we use guest_enter_irqoff() and
> > guest_exit_irqoff() directly, with interrupts masked between these. As
> > we don't handle any timer ticks during this window, we will not account
> > time spent within the guest as guest time, which is unfortunate.
> >
> > Additionally, we do not inform lockdep or tracing that interrupts will
> > be enabled during guest execution, which caan lead to misleading traces
> > and warnings that interrupts have been enabled for overly-long periods.
> >
> > This patch fixes these issues by using the new timing and context
> > entry/exit helpers to ensure that interrupts are handled during guest
> > vtime but with RCU watching, with a sequence:
> >
> > 	guest_timing_enter_irqoff();
> >
> > 	guest_state_enter_irqoff();
> > 	< run the vcpu >
> > 	guest_state_exit_irqoff();
> >
> > 	< take any pending IRQs >
> >
> > 	guest_timing_exit_irqoff();
>
> Looking again, this patch isn't sufficient.
>
> On MIPS a guest exit will be handled by kvm_mips_handle_exit() *before*
> returning into the "< run the vcpu >" step above, so we won't call
> guest_state_exit_irqoff() before using RCU, etc.

Indeed.  kvm_mips_handle_exit has a weird mutual recursion through runtime-assembled code, but then this is MIPS...

This should do it:

diff --git a/arch/mips/kvm/mips.c b/arch/mips/kvm/mips.c
index e59cb6246f76..6f2291f017f5 100644
--- a/arch/mips/kvm/mips.c
+++ b/arch/mips/kvm/mips.c
@@ -1192,6 +1192,7 @@ int kvm_mips_handle_exit(struct kvm_vcpu *vcpu)
 	kvm_mips_set_c0_status();
 
 	local_irq_enable();
+	guest_timing_exit_irqoff();
 
 	kvm_debug("kvm_mips_handle_exit: cause: %#x, PC: %p, kvm_run: %p, kvm_vcpu: %p\n",
 			cause, opc, run, vcpu);
@@ -1325,6 +1326,7 @@ int kvm_mips_handle_exit(struct kvm_vcpu *vcpu)
 	}
 
 	if (ret == RESUME_GUEST) {
+		guest_timing_enter_irqoff();
 		trace_kvm_reenter(vcpu);
 
 		/*


Paolo