linux-kernel - Re: [PATCH 3/3] virtio_blk: implement blk_mq

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Message-ID: <20210524145928.GA3873@lst.de>
Date:   Mon, 24 May 2021 16:59:28 +0200
From:   Christoph Hellwig <hch@....de>
To:     Stefan Hajnoczi <stefanha@...hat.com>
Cc:     virtualization@...ts.linux-foundation.org,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        Christoph Hellwig <hch@....de>,
        Jason Wang <jasowang@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Jens Axboe <axboe@...nel.dk>, slp@...hat.com,
        sgarzare@...hat.com, "Michael S. Tsirkin" <mst@...hat.com>
Subject: Re: [PATCH 3/3] virtio_blk: implement blk_mq_ops->poll()

On Thu, May 20, 2021 at 03:13:05PM +0100, Stefan Hajnoczi wrote:
> Possible drawbacks of this approach:
> 
> - Hardware virtio_blk implementations may find virtqueue_disable_cb()
>   expensive since it requires DMA. If such devices become popular then
>   the virtio_blk driver could use a similar approach to NVMe when
>   VIRTIO_F_ACCESS_PLATFORM is detected in the future.
> 
> - If a blk_poll() thread is descheduled it not only hurts polling
>   performance but also delays completion of non-REQ_HIPRI requests on
>   that virtqueue since vq notifications are disabled.

Yes, I think this is a dangerous configuration.  What argument exists
again just using dedicated poll queues?