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[80.230.31.118]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-432bd5ee5eesm16735262f8f.34.2026.01.08.06.17.51 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 08 Jan 2026 06:17:53 -0800 (PST) Date: Thu, 8 Jan 2026 09:17:49 -0500 From: "Michael S. Tsirkin" To: Stefano Garzarella Cc: linux-kernel@vger.kernel.org, Cong Wang , Jonathan Corbet , Olivia Mackall , Herbert Xu , Jason Wang , Paolo Bonzini , Stefan Hajnoczi , Eugenio =?iso-8859-1?Q?P=E9rez?= , "James E.J. Bottomley" , "Martin K. Petersen" , Gerd Hoffmann , Xuan Zhuo , Marek Szyprowski , Robin Murphy , "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 13/15] vsock/virtio: reorder fields to reduce padding Message-ID: <20260108091514-mutt-send-email-mst@kernel.org> References: 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: On Thu, Jan 08, 2026 at 03:11:36PM +0100, Stefano Garzarella wrote: > On Mon, Jan 05, 2026 at 03:23:41AM -0500, Michael S. Tsirkin wrote: > > Reorder struct virtio_vsock fields to place the DMA buffer (event_list) > > last. This eliminates the padding from aligning the struct size on > > ARCH_DMA_MINALIGN. > > > > Signed-off-by: Michael S. Tsirkin > > --- > > net/vmw_vsock/virtio_transport.c | 8 +++++--- > > 1 file changed, 5 insertions(+), 3 deletions(-) > > > > diff --git a/net/vmw_vsock/virtio_transport.c b/net/vmw_vsock/virtio_transport.c > > index ef983c36cb66..964d25e11858 100644 > > --- a/net/vmw_vsock/virtio_transport.c > > +++ b/net/vmw_vsock/virtio_transport.c > > @@ -60,9 +60,7 @@ struct virtio_vsock { > > */ > > struct mutex event_lock; > > bool event_run; > > - __dma_from_device_group_begin(); > > - struct virtio_vsock_event event_list[8]; > > - __dma_from_device_group_end(); > > + > > u32 guest_cid; > > bool seqpacket_allow; > > > > @@ -76,6 +74,10 @@ struct virtio_vsock { > > */ > > struct scatterlist *out_sgs[MAX_SKB_FRAGS + 1]; > > struct scatterlist out_bufs[MAX_SKB_FRAGS + 1]; > > + > > IIUC we would like to have these fields always on the bottom of this struct, > so would be better to add a comment here to make sure we will not add other > fields in the future after this? not necessarily - you can add fields after, too - it's just that __dma_from_device_group_begin already adds a bunch of padding, so adding fields in this padding is cheaper. do we really need to add comments to teach people about the art of struct packing? > Maybe we should also add a comment about the `event_lock` requirement we > have in the section above. > > Thanks, > Stefano hmm which requirement do you mean? > > > + __dma_from_device_group_begin(); > > + struct virtio_vsock_event event_list[8]; > > + __dma_from_device_group_end(); > > }; > > > > static u32 virtio_transport_get_local_cid(void) > > -- > > MST > >