linux-kernel - Re: [PATCH v2 13/15] vsock/virtio: reorder fields to reduce padding

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Message-ID: <aV_Cr_f47qqc2JoP@sgarzare-redhat>
Date: Thu, 8 Jan 2026 15:45:20 +0100
From: Stefano Garzarella <sgarzare@...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>, Stefan Hajnoczi <stefanha@...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>, "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 13/15] vsock/virtio: reorder fields to reduce padding

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 <mst@...hat.com>
>> > > > ---
>> > > > 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