Re: [PATCH] KVM: Ensure the exit frequency to QEmu for coalesced MMIO

[Sheng Yang <sheng@linux.intel.com>]

On Wednesday 20 January 2010 16:35:59 Sheng Yang wrote:
> The default action of calesced MMIO is, cache the writing in buffer, until:
> 1. The buffer is full.
> 2. Or the exit to QEmu due to other reasons.
> 
> But this would result in a very late writing in some condition.
> 1. The each time write to MMIO content is small.
> 2. The writing interval is big.
> 3. No need for input or accessing other devices frequently.

"No need for exiting to the QEmu in a period" should be more precise...

-- 
regards
Yang, Sheng

> 
> This issue was observed in a experimental embbed system. The test image
>  simply print "test" every 1 seconds. The output in QEmu meets expectation,
>  but the output in KVM is delayed for seconds.
> 
> To solve this issue, a timeout is added, to ensure userspace exit freqency
>  is high enough(mostly target for video now) to write the buffered MMIO
>  data. Current the maximum exit interval is 1/25s(so 25 times exit to
>  userspace per second at least, pretty low compared to normal environment),
>  and reused KVM_EXIT_IRQ_WINDOW_OPEN as exit reason, for it would doing
>  nothing at all in userspace handler.
> 
> Signed-off-by: Sheng Yang <sheng@linux.intel.com>
> ---
>  arch/x86/include/asm/kvm_host.h |    2 ++
>  arch/x86/kvm/x86.c              |   31 +++++++++++++++++++++++++++++++
>  2 files changed, 33 insertions(+), 0 deletions(-)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h
>  b/arch/x86/include/asm/kvm_host.h index a1f0b5d..eb8bb20 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -365,6 +365,8 @@ struct kvm_vcpu_arch {
>  	unsigned long singlestep_rip;
>  	/* fields used by HYPER-V emulation */
>  	u64 hv_vapic;
> +
> +	ktime_t latest_userspace_exit_time;
>  };
> 
>  struct kvm_mem_alias {
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 56a90a6..0b05f11 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -4339,6 +4339,20 @@ out:
>  	return r;
>  }
> 
> +#ifdef CONFIG_KVM_MMIO
> +
> +#define KVM_USERSPACE_MMIO_MAX_INTERVAL (NSEC_PER_SEC / 25)
> +static bool mmio_need_exit_to_userspace(struct kvm_vcpu *vcpu)
> +{
> +	ktime_t gap, now = ktime_get();
> +
> +	gap = ktime_sub(now, vcpu->arch.latest_userspace_exit_time);
> +	if (ktime_to_ns(gap) > KVM_USERSPACE_MMIO_MAX_INTERVAL)
> +		return true;
> +
> +	return false;
> +}
> +#endif
> 
>  static int __vcpu_run(struct kvm_vcpu *vcpu)
>  {
> @@ -4404,6 +4418,10 @@ static int __vcpu_run(struct kvm_vcpu *vcpu)
>  			kvm_resched(vcpu);
>  			vcpu->srcu_idx = srcu_read_lock(&kvm->srcu);
>  		}
> +#ifdef CONFIG_KVM_MMIO
> +		if (mmio_need_exit_to_userspace(vcpu))
> +			r = 0;
> +#endif
>  	}
> 
>  	srcu_read_unlock(&kvm->srcu, vcpu->srcu_idx);
> @@ -4463,6 +4481,16 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu,
>  struct kvm_run *kvm_run)
> 
>  	r = __vcpu_run(vcpu);
> 
> +#ifdef CONFIG_KVM_MMIO
> +	if (mmio_need_exit_to_userspace(vcpu)) {
> +		/* Use KVM_EXIT_IRQ_WINDOW_OPEN because userspace would do
> +		 * nothing to handle it */
> +		kvm_run->exit_reason = KVM_EXIT_IRQ_WINDOW_OPEN;
> +		r = 0;
> +	}
> +	vcpu->arch.latest_userspace_exit_time = ktime_get();
> +#endif
> +
>  out:
>  	if (vcpu->sigset_active)
>  		sigprocmask(SIG_SETMASK, &sigsaved, NULL);
> @@ -5455,6 +5483,9 @@ int kvm_arch_vcpu_init(struct kvm_vcpu *vcpu)
>  		goto fail_mmu_destroy;
>  	}
>  	vcpu->arch.mcg_cap = KVM_MAX_MCE_BANKS;
> +#ifdef CONFIG_KVM_MMIO
> +	vcpu->arch.latest_userspace_exit_time = ktime_get();
> +#endif
> 
>  	return 0;
>