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[80.230.31.118]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-47d8717d78fsm38131185e9.9.2026.01.08.06.32.25 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 08 Jan 2026 06:32:27 -0800 (PST) Date: Thu, 8 Jan 2026 09:32:23 -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: <20260108092931-mutt-send-email-mst@kernel.org> References: <20260108091514-mutt-send-email-mst@kernel.org> 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 03:27:04PM +0100, Stefano Garzarella wrote: > On Thu, Jan 08, 2026 at 09:17:49AM -0500, Michael S. Tsirkin wrote: > > 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. > > > > Okay, I see. > > > > > do we really need to add comments to teach people about the art of > > struct packing? > > I can do it later if you prefer, I don't want to block this work, but yes, > I'd prefer to have a comment because otherwise I'll have to ask every time > to avoid, especially for new contributors xD On the one hand you are right on the other I don't want it duplicated each time __dma_from_device_group_begin is invoked. Pls come up with something you like, and we'll discuss. > > > > > Maybe we should also add a comment about the `ev`nt_lock` > > > requirement we > > > have in the section above. > > > > > > Thanks, > > > Stefano > > > > hmm which requirement do you mean? > > That `event_list` must be accessed with `event_lock`. > > So maybe we can move also `event_lock` and `event_run`, so we can just move > that comment. I mean something like this: > > > @@ -74,6 +67,15 @@ struct virtio_vsock { > */ > struct scatterlist *out_sgs[MAX_SKB_FRAGS + 1]; > struct scatterlist out_bufs[MAX_SKB_FRAGS + 1]; > + > + /* The following fields are protected by event_lock. > + * vqs[VSOCK_VQ_EVENT] must be accessed with event_lock held. > + */ > + struct mutex event_lock; > + bool event_run; > + __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) Yea this makes sense. > > Thanks, > Stefano