Re: [PATCH 2/2] KVM: Call kvm_arch_vcpu_blocking early into the blocking sequence

[Marc Zyngier <maz@kernel.org>]

On Fri, 2 Aug 2019 12:46:33 +0200
Paolo Bonzini <pbonzini@redhat.com> wrote:

> On 02/08/19 12:37, Marc Zyngier wrote:
> > When a vpcu is about to block by calling kvm_vcpu_block, we call
> > back into the arch code to allow any form of synchronization that
> > may be required at this point (SVN stops the AVIC, ARM synchronises
> > the VMCR and enables GICv4 doorbells). But this synchronization
> > comes in quite late, as we've potentially waited for halt_poll_ns
> > to expire.
> > 
> > Instead, let's move kvm_arch_vcpu_blocking() to the beginning of
> > kvm_vcpu_block(), which on ARM has several benefits:
> > 
> > - VMCR gets synchronised early, meaning that any interrupt delivered
> >   during the polling window will be evaluated with the correct guest
> >   PMR
> > - GICv4 doorbells are enabled, which means that any guest interrupt
> >   directly injected during that window will be immediately recognised
> > 
> > Tang Nianyao ran some tests on a GICv4 machine to evaluate such
> > change, and reported up to a 10% improvement for netperf:
> > 
> > <quote>
> > 	netperf result:
> > 	D06 as server, intel 8180 server as client
> > 	with change:
> > 	package 512 bytes - 5500 Mbits/s
> > 	package 64 bytes - 760 Mbits/s
> > 	without change:
> > 	package 512 bytes - 5000 Mbits/s
> > 	package 64 bytes - 710 Mbits/s
> > </quote>
> > 
> > Signed-off-by: Marc Zyngier <maz@kernel.org>
> > ---
> >  virt/kvm/kvm_main.c | 7 +++----
> >  1 file changed, 3 insertions(+), 4 deletions(-)
> > 
> > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > index 887f3b0c2b60..90d429c703cb 100644
> > --- a/virt/kvm/kvm_main.c
> > +++ b/virt/kvm/kvm_main.c
> > @@ -2322,6 +2322,8 @@ void kvm_vcpu_block(struct kvm_vcpu *vcpu)
> >  	bool waited = false;
> >  	u64 block_ns;
> >  
> > +	kvm_arch_vcpu_blocking(vcpu);
> > +
> >  	start = cur = ktime_get();
> >  	if (vcpu->halt_poll_ns && !kvm_arch_no_poll(vcpu)) {
> >  		ktime_t stop = ktime_add_ns(ktime_get(), vcpu->halt_poll_ns);
> > @@ -2342,8 +2344,6 @@ void kvm_vcpu_block(struct kvm_vcpu *vcpu)
> >  		} while (single_task_running() && ktime_before(cur, stop));
> >  	}
> >  
> > -	kvm_arch_vcpu_blocking(vcpu);
> > -
> >  	for (;;) {
> >  		prepare_to_swait_exclusive(&vcpu->wq, &wait, TASK_INTERRUPTIBLE);
> >  
> > @@ -2356,9 +2356,8 @@ void kvm_vcpu_block(struct kvm_vcpu *vcpu)
> >  
> >  	finish_swait(&vcpu->wq, &wait);
> >  	cur = ktime_get();
> > -
> > -	kvm_arch_vcpu_unblocking(vcpu);
> >  out:
> > +	kvm_arch_vcpu_unblocking(vcpu);
> >  	block_ns = ktime_to_ns(cur) - ktime_to_ns(start);
> >  
> >  	if (!vcpu_valid_wakeup(vcpu))
> >   
> 
> Acked-by: Paolo Bonzini <pbonzini@redhat.com>

Thanks for that. I've pushed this patch into -next so that it gets a
bit of exposure (I haven't heard from the AMD folks, and I'd like to
make sure it doesn't regress their platforms).

	M.
-- 
Without deviation from the norm, progress is not possible.