Re: [External] Re: 9p regression (Was: [PATCH v2] virtio_ring: don't update event idx on get_buf)

From: "Michael S. Tsirkin" <mst@redhat.com>
To: Jason Wang <jasowang@redhat.com>
Cc: 黄杰 <huangjie.albert@bytedance.com>,
	"Christian Schoenebeck" <linux_oss@crudebyte.com>,
	linux-kernel@vger.kernel.org,
	virtualization@lists.linux-foundation.org,
	"Eric Van Hensbergen" <ericvh@gmail.com>,
	"Luis Chamberlain" <mcgrof@kernel.org>,
	v9fs-developer@lists.sourceforge.net
Subject: Re: [External] Re: 9p regression (Was: [PATCH v2] virtio_ring: don't update event idx on get_buf)
Date: Wed, 29 Mar 2023 01:21:42 -0400	[thread overview]
Message-ID: <20230329012038-mutt-send-email-mst@kernel.org> (raw)
In-Reply-To: <CACGkMEuukvjXBTDX2K9YLYmpHsqK96AiMK39gbm3+f_+kUydMQ@mail.gmail.com>

On Tue, Mar 28, 2023 at 11:39:59AM +0800, Jason Wang wrote:
> On Tue, Mar 28, 2023 at 11:09 AM 黄杰 <huangjie.albert@bytedance.com> wrote:
> >
> > Jason Wang <jasowang@redhat.com> 于2023年3月28日周二 10:59写道：
> > >
> > > On Tue, Mar 28, 2023 at 10:13 AM Dominique Martinet
> > > <asmadeus@codewreck.org> wrote:
> > > >
> > > > Hi Michael, Albert,
> > > >
> > > > Albert Huang wrote on Sat, Mar 25, 2023 at 06:56:33PM +0800:
> > > > > in virtio_net, if we disable the napi_tx, when we triger a tx interrupt,
> > > > > the vq->event_triggered will be set to true. It will no longer be set to
> > > > > false. Unless we explicitly call virtqueue_enable_cb_delayed or
> > > > > virtqueue_enable_cb_prepare.
> > > >
> > > > This patch (commited as 35395770f803 ("virtio_ring: don't update event
> > > > idx on get_buf") in next-20230327 apparently breaks 9p, as reported by
> > > > Luis in https://lkml.kernel.org/r/ZCI+7Wg5OclSlE8c@bombadil.infradead.org
> > > >
> > > > I've just hit had a look at recent patches[1] and reverted this to test
> > > > and I can mount again, so I'm pretty sure this is the culprit, but I
> > > > didn't look at the content at all yet so cannot advise further.
> > > > It might very well be that we need some extra handling for 9p
> > > > specifically that can be added separately if required.
> > > >
> > > > [1] git log 0ec57cfa721fbd36b4c4c0d9ccc5d78a78f7fa35..HEAD drivers/virtio/
> > > >
> > > >
> > > > This can be reproduced with a simple mount, run qemu with some -virtfs
> > > > argument and `mount -t 9p -o debug=65535 tag mountpoint` will hang after
> > > > these messages:
> > > > 9pnet: -- p9_virtio_request (83): 9p debug: virtio request
> > > > 9pnet: -- p9_virtio_request (83): virtio request kicked
> > > >
> > > > So I suspect we're just not getting a callback.
> > >
> > > I think so. The patch assumes the driver will call
> > > virtqueue_disable/enable_cb() which is not the case of the 9p driver.
> > >
> > > So after the first interrupt, event_triggered will be set to true forever.
> > >
> > > Thanks
> > >
> >
> > Hi: Wang
> >
> > Yes,  This patch assumes that all virtio-related drivers will call
> > virtqueue_disable/enable_cb().
> > Thank you for raising this issue.
> >
> > It seems that napi_tx is only related to virtue_net. I'm thinking if
> > we need to refactor
> > napi_tx instead of implementing it inside virtio_ring.
> 
> We can hear from others.
> 
> I think it's better not to workaround virtio_ring issues in a specific
> driver. It might just add more hacks. We should correctly set
> VRING_AVAIL_F_NO_INTERRUPT,

I am still stuck trying to understand why we don't set it.
How does event_triggered end up getting set without
event index support?

Thanks!

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