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From:   Jason Wang <jasowang@redhat.com>
Date:   Wed, 17 May 2023 13:23:50 +0800
Message-ID: <CACGkMEtitFX1v=fFYohLNz=xo3S7CM3Cdt09=C6xXz8kb1a4Cg@mail.gmail.com>
Subject: Re: [PATCH v1] virtio_pci: Optimize virtio_pci_device structure size
To:     Feng Liu <feliu@nvidia.com>
Cc:     virtualization@lists.linux-foundation.org, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org,
        "Michael S . Tsirkin" <mst@redhat.com>,
        Xuan Zhuo <xuanzhuo@linux.alibaba.com>,
        Bodong Wang <bodong@nvidia.com>, Jiri Pirko <jiri@nvidia.com>
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On Tue, May 16, 2023 at 9:55=E2=80=AFPM Feng Liu <feliu@nvidia.com> wrote:
>
> Improve the size of the virtio_pci_device structure, which is commonly
> used to represent a virtio PCI device. A given virtio PCI device can
> either of legacy type or modern type, with the
> struct virtio_pci_legacy_device occupying 32 bytes and the
> struct virtio_pci_modern_device occupying 88 bytes. Make them a union,
> thereby save 32 bytes of memory as shown by the pahole tool. This
> improvement is particularly beneficial when dealing with numerous
> devices, as it helps conserve memory resources.
>
> Before the modification, pahole tool reported the following:
> struct virtio_pci_device {
> [...]
>         struct virtio_pci_legacy_device ldev;            /*   824    32 *=
/
>         /* --- cacheline 13 boundary (832 bytes) was 24 bytes ago --- */
>         struct virtio_pci_modern_device mdev;            /*   856    88 *=
/
>
>         /* XXX last struct has 4 bytes of padding */
> [...]
>         /* size: 1056, cachelines: 17, members: 19 */
> [...]
> };
>
> After the modification, pahole tool reported the following:
> struct virtio_pci_device {
> [...]
>         union {
>                 struct virtio_pci_legacy_device ldev;    /*   824    32 *=
/
>                 struct virtio_pci_modern_device mdev;    /*   824    88 *=
/
>         };                                               /*   824    88 *=
/
> [...]
>         /* size: 1024, cachelines: 16, members: 18 */
> [...]
> };
>
> Signed-off-by: Feng Liu <feliu@nvidia.com>
> Reviewed-by: Jiri Pirko <jiri@nvidia.com>

Acked-by: Jason Wang <jasowang@redhat.com>

Thanks

> ---
>  drivers/virtio/virtio_pci_common.h | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/virtio/virtio_pci_common.h b/drivers/virtio/virtio_p=
ci_common.h
> index 23112d84218f..4b773bd7c58c 100644
> --- a/drivers/virtio/virtio_pci_common.h
> +++ b/drivers/virtio/virtio_pci_common.h
> @@ -45,9 +45,10 @@ struct virtio_pci_vq_info {
>  struct virtio_pci_device {
>         struct virtio_device vdev;
>         struct pci_dev *pci_dev;
> -       struct virtio_pci_legacy_device ldev;
> -       struct virtio_pci_modern_device mdev;
> -
> +       union {
> +               struct virtio_pci_legacy_device ldev;
> +               struct virtio_pci_modern_device mdev;
> +       };
>         bool is_legacy;
>
>         /* Where to read and clear interrupt */
> --
> 2.37.1 (Apple Git-137.1)
>