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

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Message-ID: <20260107162939.GA193868@fedora>
Date: Wed, 7 Jan 2026 11:29:39 -0500
From: Stefan Hajnoczi <stefanha@...hat.com>
To: "Michael S. Tsirkin" <mst@...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 Tue, Jan 06, 2026 at 09:50:00AM -0500, Michael S. Tsirkin wrote:
> 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?
> 
> No because virtio does unmap and syncs the cache line.
> 
> In other words, CPU reads cause no issues.
> 
> The issues are exclusively around CPU writes dirtying the
> cache and writeback overwriting DMA data.

I see. In that case I'm happy with the virtio-scsi change:

Reviewed-by: Stefan Hajnoczi <stefanha@...hat.com>

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