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SubjectRe: [PATCH] x86/kvm/hyper-v: avoid spurious pending stimer on vCPU init
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On 13/03/19 18:13, Vitaly Kuznetsov wrote:
> When userspace initializes guest vCPUs it may want to zero all supported
> MSRs including Hyper-V related ones including HV_X64_MSR_STIMERn_CONFIG/
> HV_X64_MSR_STIMERn_COUNT. With commit f3b138c5d89a ("kvm/x86: Update SynIC
> timers on guest entry only") we began doing stimer_mark_pending()
> unconditionally on every config change.
>
> The issue I'm observing manifests itself as following:
> - Qemu writes 0 to STIMERn_{CONFIG,COUNT} MSRs and marks all stimers as
> pending in stimer_pending_bitmap, arms KVM_REQ_HV_STIMER;
> - kvm_hv_has_stimer_pending() starts returning true;
> - kvm_vcpu_has_events() starts returning true;
> - kvm_arch_vcpu_runnable() starts returning true;
> - when kvm_arch_vcpu_ioctl_run() gets into
> (vcpu->arch.mp_state == KVM_MP_STATE_UNINITIALIZED) case:
> - kvm_vcpu_block() gets in 'kvm_vcpu_check_block(vcpu) < 0' and returns
> immediately, avoiding normal wait path;
> - -EAGAIN is returned from kvm_arch_vcpu_ioctl_run() immediately forcing
> userspace to retry.
>
> So instead of normal wait path we get a busy loop on all secondary vCPUs
> before they get INIT signal. This seems to be undesirable, especially given
> that this happens even when Hyper-V extensions are not used.
>
> Generally, it seems to be pointless to mark an stimer as pending in
> stimer_pending_bitmap and arm KVM_REQ_HV_STIMER as the only thing
> kvm_hv_process_stimers() will do is clear the corresponding bit. We may
> just not mark disabled timers as pending instead.
>
> Fixes: f3b138c5d89a ("kvm/x86: Update SynIC timers on guest entry only")
> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
> ---
> arch/x86/kvm/hyperv.c | 9 +++++++--
> 1 file changed, 7 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
> index 89d20ed1d2e8..371c669696d7 100644
> --- a/arch/x86/kvm/hyperv.c
> +++ b/arch/x86/kvm/hyperv.c
> @@ -526,7 +526,9 @@ static int stimer_set_config(struct kvm_vcpu_hv_stimer *stimer, u64 config,
> new_config.enable = 0;
> stimer->config.as_uint64 = new_config.as_uint64;
>
> - stimer_mark_pending(stimer, false);
> + if (stimer->config.enable)
> + stimer_mark_pending(stimer, false);
> +
> return 0;
> }
>
> @@ -542,7 +544,10 @@ static int stimer_set_count(struct kvm_vcpu_hv_stimer *stimer, u64 count,
> stimer->config.enable = 0;
> else if (stimer->config.auto_enable)
> stimer->config.enable = 1;
> - stimer_mark_pending(stimer, false);
> +
> + if (stimer->config.enable)
> + stimer_mark_pending(stimer, false);
> +
> return 0;
> }
>
>

Queued for after the merge window.

Paolo

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