linux-kernel - Re: [PATCH v2 04/10] vhost: modify vhost_log

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Message-ID: <CACGkMEtOsQg68O+Nqo9ycLSq7sN4AMZ92ZvLLMEF7xYDCA5Ycw@mail.gmail.com>
Date: Tue, 18 Mar 2025 09:12:53 +0800
From: Jason Wang <jasowang@...hat.com>
To: Dongli Zhang <dongli.zhang@...cle.com>
Cc: virtualization@...ts.linux.dev, kvm@...r.kernel.org, 
	netdev@...r.kernel.org, mst@...hat.com, michael.christie@...cle.com, 
	pbonzini@...hat.com, stefanha@...hat.com, eperezma@...hat.com, 
	joao.m.martins@...cle.com, joe.jin@...cle.com, si-wei.liu@...cle.com, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 04/10] vhost: modify vhost_log_write() for broader users

On Tue, Mar 18, 2025 at 7:51 AM Dongli Zhang <dongli.zhang@...cle.com> wrote:
>
> Currently, the only user of vhost_log_write() is vhost-net. The 'len'
> argument prevents logging of pages that are not tainted by the RX path.
>
> Adjustments are needed since more drivers (i.e. vhost-scsi) begin using
> vhost_log_write(). So far vhost-net RX path may only partially use pages
> shared by the last vring descriptor. Unlike vhost-net, vhost-scsi always
> logs all pages shared via vring descriptors. To accommodate this, a new
> argument 'partial' is introduced. This argument works alongside 'len' to
> indicate whether the driver should log all pages of a vring descriptor, or
> only pages that are tainted by the driver.
>
> In addition, removes BUG().
>
> Suggested-by: Joao Martins <joao.m.martins@...cle.com>
> Signed-off-by: Dongli Zhang <dongli.zhang@...cle.com>
> ---
>  drivers/vhost/net.c   |  2 +-
>  drivers/vhost/vhost.c | 28 +++++++++++++++++-----------
>  drivers/vhost/vhost.h |  2 +-
>  3 files changed, 19 insertions(+), 13 deletions(-)
>
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index b9b9e9d40951..0e5d82bfde76 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -1219,7 +1219,7 @@ static void handle_rx(struct vhost_net *net)
>                 if (nvq->done_idx > VHOST_NET_BATCH)
>                         vhost_net_signal_used(nvq);
>                 if (unlikely(vq_log))
> -                       vhost_log_write(vq, vq_log, log, vhost_len,
> +                       vhost_log_write(vq, vq_log, log, vhost_len, true,
>                                         vq->iov, in);
>                 total_len += vhost_len;
>         } while (likely(!vhost_exceeds_weight(vq, ++recv_pkts, total_len)));
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index 9ac25d08f473..db3b30aba940 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -2304,8 +2304,14 @@ static int log_used(struct vhost_virtqueue *vq, u64 used_offset, u64 len)
>         return 0;
>  }
>
> -int vhost_log_write(struct vhost_virtqueue *vq, struct vhost_log *log,
> -                   unsigned int log_num, u64 len, struct iovec *iov, int count)
> +/*
> + * 'len' is used only when 'partial' is true, to indicate whether the
> + * entire length of each descriptor is logged.
> + */

While at it, let's document all the parameters here.

> +int vhost_log_write(struct vhost_virtqueue *vq,
> +                   struct vhost_log *log, unsigned int log_num,
> +                   u64 len, bool partial,
> +                   struct iovec *iov, int count)
>  {
>         int i, r;
>
> @@ -2323,19 +2329,19 @@ int vhost_log_write(struct vhost_virtqueue *vq, struct vhost_log *log,
>         }
>
>         for (i = 0; i < log_num; ++i) {
> -               u64 l = min(log[i].len, len);
> +               u64 l = partial ? min(log[i].len, len) : log[i].len;
> +
>                 r = log_write(vq->log_base, log[i].addr, l);
>                 if (r < 0)
>                         return r;
> -               len -= l;
> -               if (!len) {
> -                       if (vq->log_ctx)
> -                               eventfd_signal(vq->log_ctx);
> -                       return 0;
> -               }
> +
> +               if (partial)
> +                       len -= l;

I wonder if it's simpler to just tweak the caller to call with the
correct len (or probably U64_MAX) in this case?

>         }
> -       /* Length written exceeds what we have stored. This is a bug. */
> -       BUG();
> +
> +       if (vq->log_ctx)
> +               eventfd_signal(vq->log_ctx);
> +
>         return 0;
>  }
>  EXPORT_SYMBOL_GPL(vhost_log_write);
> diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
> index bb75a292d50c..5de5941988fe 100644
> --- a/drivers/vhost/vhost.h
> +++ b/drivers/vhost/vhost.h
> @@ -224,7 +224,7 @@ bool vhost_vq_avail_empty(struct vhost_dev *, struct vhost_virtqueue *);
>  bool vhost_enable_notify(struct vhost_dev *, struct vhost_virtqueue *);
>
>  int vhost_log_write(struct vhost_virtqueue *vq, struct vhost_log *log,
> -                   unsigned int log_num, u64 len,
> +                   unsigned int log_num, u64 len, bool partial,
>                     struct iovec *iov, int count);
>  int vq_meta_prefetch(struct vhost_virtqueue *vq);
>
> --
> 2.39.3
>

Thanks