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@xxxxxxxxxx>
> ---
> 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@xxxxxxxxxx>
