linux-kernel - Re: [PATCH v3 4/9] vhost: modify vhost_log

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Message-ID: <3cf501d5-925c-4a8c-b3cc-d0b94b4b5e8a@oracle.com>
Date: Wed, 16 Apr 2025 00:58:05 -0700
From: Dongli Zhang <dongli.zhang@...cle.com>
To: virtualization@...ts.linux.dev, kvm@...r.kernel.org,
        netdev@...r.kernel.org, jasowang@...hat.com
Cc: 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 v3 4/9] vhost: modify vhost_log_write() for broader users

Hi Jason,

Would you mind helping confirm if it resolves your concern in v2?

Thank you very much!

Dongli Zhang

On 4/2/25 11:29 PM, Dongli Zhang 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 via the last vring descriptor. Unlike vhost-net, vhost-scsi always
> logs all pages shared via vring descriptors. To accommodate this,
> use (len == U64_MAX) to indicate whether the driver would log all pages of
> vring descriptors, 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>
> ---
> Changed since v2:
>   - Document parameters of vhost_log_write().
>   - Use (len == U64_MAX) to indicate whether log all pages, instead of
>     introducing a new parameter.
> 
>  drivers/vhost/vhost.c | 28 ++++++++++++++++++++--------
>  1 file changed, 20 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index 9ac25d08f473..494b3da5423a 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -2304,6 +2304,19 @@ static int log_used(struct vhost_virtqueue *vq, u64 used_offset, u64 len)
>  	return 0;
>  }
>  
> +/*
> + * vhost_log_write() - Log in dirty page bitmap
> + * @vq:      vhost virtqueue.
> + * @log:     Array of dirty memory in GPA.
> + * @log_num: Size of vhost_log arrary.
> + * @len:     The total length of memory buffer to log in the dirty bitmap.
> + *	     Some drivers may only partially use pages shared via the last
> + *	     vring descriptor (i.e. vhost-net RX buffer).
> + *	     Use (len == U64_MAX) to indicate the driver would log all
> + *           pages of vring descriptors.
> + * @iov:     Array of dirty memory in HVA.
> + * @count:   Size of iovec array.
> + */
>  int vhost_log_write(struct vhost_virtqueue *vq, struct vhost_log *log,
>  		    unsigned int log_num, u64 len, struct iovec *iov, int count)
>  {
> @@ -2327,15 +2340,14 @@ int vhost_log_write(struct vhost_virtqueue *vq, struct vhost_log *log,
>  		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 (len != U64_MAX)
> +			len -= l;
>  	}
> -	/* 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);