linux-kernel - Re: [RFC PATCH 2/2] kvm/eventfd: Use priority waitqueue to catch events before userspace

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Message-ID: <1faa5405-3640-f4ad-5cd9-89a9e5e834e9@redhat.com>
Date:   Tue, 27 Oct 2020 09:01:51 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     David Woodhouse <dwmw2@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [RFC PATCH 2/2] kvm/eventfd: Use priority waitqueue to catch
 events before userspace

On 26/10/20 18:53, David Woodhouse wrote:
> From: David Woodhouse <dwmw@...zon.co.uk>
> 
> As far as I can tell, when we use posted interrupts we silently cut off
> the events from userspace, if it's listening on the same eventfd that
> feeds the irqfd.
> 
> I like that behaviour. Let's do it all the time, even without posted
> interrupts. It makes it much easier to handle IRQ remapping invalidation
> without having to constantly add/remove the fd from the userspace poll
> set. We can just leave userspace polling on it, and the bypass will...
> well... bypass it.

This looks good, though of course it depends on the somewhat hackish
patch 1. However don't you need to read the eventfd as well, since
userspace will never be able to do so?

Paolo

> Signed-off-by: David Woodhouse <dwmw@...zon.co.uk>
> ---
>  virt/kvm/eventfd.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/virt/kvm/eventfd.c b/virt/kvm/eventfd.c
> index d6408bb497dc..39443e2f72bf 100644
> --- a/virt/kvm/eventfd.c
> +++ b/virt/kvm/eventfd.c
> @@ -191,6 +191,7 @@ irqfd_wakeup(wait_queue_entry_t *wait, unsigned mode, int sync, void *key)
>  	struct kvm *kvm = irqfd->kvm;
>  	unsigned seq;
>  	int idx;
> +	int ret = 0;
>  
>  	if (flags & EPOLLIN) {
>  		idx = srcu_read_lock(&kvm->irq_srcu);
> @@ -204,6 +205,7 @@ irqfd_wakeup(wait_queue_entry_t *wait, unsigned mode, int sync, void *key)
>  					      false) == -EWOULDBLOCK)
>  			schedule_work(&irqfd->inject);
>  		srcu_read_unlock(&kvm->irq_srcu, idx);
> +		ret = 1;
>  	}
>  
>  	if (flags & EPOLLHUP) {
> @@ -227,7 +229,7 @@ irqfd_wakeup(wait_queue_entry_t *wait, unsigned mode, int sync, void *key)
>  		spin_unlock_irqrestore(&kvm->irqfds.lock, iflags);
>  	}
>  
> -	return 0;
> +	return ret;
>  }
>  
>  static void
> @@ -236,7 +238,7 @@ irqfd_ptable_queue_proc(struct file *file, wait_queue_head_t *wqh,
>  {
>  	struct kvm_kernel_irqfd *irqfd =
>  		container_of(pt, struct kvm_kernel_irqfd, pt);
> -	add_wait_queue(wqh, &irqfd->wait);
> +	add_wait_queue_priority(wqh, &irqfd->wait);
>  }
>  
>  /* Must be called under irqfds.lock */
>