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Thu, 08 Jan 2026 06:45:29 -0800 (PST) Received: from sgarzare-redhat ([193.207.223.215]) by smtp.gmail.com with ESMTPSA id a640c23a62f3a-b842a564255sm848076466b.63.2026.01.08.06.45.26 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 08 Jan 2026 06:45:28 -0800 (PST) Date: Thu, 8 Jan 2026 15:45:20 +0100 From: Stefano Garzarella To: "Michael S. Tsirkin" Cc: linux-kernel@vger.kernel.org, Cong Wang , Jonathan Corbet , Olivia Mackall , Herbert Xu , Jason Wang , Paolo Bonzini , Stefan Hajnoczi , Eugenio =?utf-8?B?UMOpcmV6?= , "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: References: <20260108091514-mutt-send-email-mst@kernel.org> <20260108092931-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; format=flowed Content-Disposition: inline In-Reply-To: <20260108092931-mutt-send-email-mst@kernel.org> On Thu, Jan 08, 2026 at 09:32:23AM -0500, Michael S. Tsirkin wrote: >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. yeah, I see. >Pls come up with something you like, and we'll discuss. sure, I'll check a bit similar cases to have some inspiration. > >> > >> > > 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 for that! Stefano