netdev - Re: [PATCH v2 10/15] virtio_scsi: fix DMA cacheline issues for events

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Message-ID: <20260106095044-mutt-send-email-mst@kernel.org>
Date: Tue, 6 Jan 2026 09:51:46 -0500
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Stefan Hajnoczi <stefanha@...hat.com>
Cc: linux-kernel@...r.kernel.org, Cong Wang <xiyou.wangcong@...il.com>,
	Jonathan Corbet <corbet@....net>,
	Olivia Mackall <olivia@...enic.com>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Jason Wang <jasowang@...hat.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Eugenio Pérez <eperezma@...hat.com>,
	"James E.J. Bottomley" <James.Bottomley@...senpartnership.com>,
	"Martin K. Petersen" <martin.petersen@...cle.com>,
	Gerd Hoffmann <kraxel@...hat.com>,
	Xuan Zhuo <xuanzhuo@...ux.alibaba.com>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Robin Murphy <robin.murphy@....com>,
	Stefano Garzarella <sgarzare@...hat.com>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Simon Horman <horms@...nel.org>, Petr Tesarik <ptesarik@...e.com>,
	Leon Romanovsky <leon@...nel.org>, Jason Gunthorpe <jgg@...pe.ca>,
	Bartosz Golaszewski <brgl@...nel.org>, linux-doc@...r.kernel.org,
	linux-crypto@...r.kernel.org, virtualization@...ts.linux.dev,
	linux-scsi@...r.kernel.org, iommu@...ts.linux.dev,
	kvm@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH v2 10/15] virtio_scsi: fix DMA cacheline issues for events

On Mon, Jan 05, 2026 at 01:19:39PM -0500, Stefan Hajnoczi wrote:
> On Mon, Jan 05, 2026 at 03:23:29AM -0500, Michael S. Tsirkin wrote:
> > @@ -61,7 +62,7 @@ struct virtio_scsi_cmd {
> >  
> >  struct virtio_scsi_event_node {
> >  	struct virtio_scsi *vscsi;
> > -	struct virtio_scsi_event event;
> > +	struct virtio_scsi_event *event;
> >  	struct work_struct work;
> >  };
> >  
> > @@ -89,6 +90,11 @@ struct virtio_scsi {
> >  
> >  	struct virtio_scsi_vq ctrl_vq;
> >  	struct virtio_scsi_vq event_vq;
> > +
> > +	__dma_from_device_group_begin();
> > +	struct virtio_scsi_event events[VIRTIO_SCSI_EVENT_LEN];
> > +	__dma_from_device_group_end();
> 
> If the device emits two events in rapid succession, could the CPU see
> stale data for the second event because it already holds the cache line
> for reading the first event?
> 
> In other words, it's not obvious to me that the DMA warnings are indeed
> spurious and should be silenced here.
> 
> It seems safer and simpler to align and pad the struct virtio_scsi_event
> field in struct virtio_scsi_event_node rather than packing these structs
> into a single array here they might share cache lines.
> 
> Stefan



To add to what I wrote, that's a lot of overhead: 8 * 128 - about 1K on
some platforms, and these happen to be low end ones.

-- 
MST