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[80.230.31.118]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-432bd5ede7esm4752453f8f.32.2026.01.06.06.51.48 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 06 Jan 2026 06:51:51 -0800 (PST) Date: Tue, 6 Jan 2026 09:51:46 -0500 From: "Michael S. Tsirkin" To: Stefan Hajnoczi Cc: linux-kernel@vger.kernel.org, Cong Wang , Jonathan Corbet , Olivia Mackall , Herbert Xu , Jason Wang , Paolo Bonzini , Eugenio =?iso-8859-1?Q?P=E9rez?= , "James E.J. Bottomley" , "Martin K. Petersen" , Gerd Hoffmann , Xuan Zhuo , Marek Szyprowski , Robin Murphy , Stefano Garzarella , "David S. Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Simon Horman , Petr Tesarik , Leon Romanovsky , Jason Gunthorpe , Bartosz Golaszewski , linux-doc@vger.kernel.org, linux-crypto@vger.kernel.org, virtualization@lists.linux.dev, linux-scsi@vger.kernel.org, iommu@lists.linux.dev, kvm@vger.kernel.org, netdev@vger.kernel.org Subject: Re: [PATCH v2 10/15] virtio_scsi: fix DMA cacheline issues for events Message-ID: <20260106095044-mutt-send-email-mst@kernel.org> References: <8801aeef7576a155299f19b6887682dd3a272aba.1767601130.git.mst@redhat.com> <20260105181939.GA59391@fedora> Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260105181939.GA59391@fedora> 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