Re: [PATCH v3] virtio_blk: add VIRTIO_BLK_F_LIFETIME feature support

From: "Michael S. Tsirkin" <mst@redhat.com>
To: Chaitanya Kulkarni <chaitanyak@nvidia.com>
Cc: Jens Axboe <axboe@kernel.dk>,
	Christoph Hellwig <hch@infradead.org>,
	"linux-scsi@vger.kernel.org" <linux-scsi@vger.kernel.org>,
	"linux-nvme@lists.infradead.org" <linux-nvme@lists.infradead.org>,
	"virtualization@lists.linux-foundation.org" 
	<virtualization@lists.linux-foundation.org>,
	"linux-block@vger.kernel.org" <linux-block@vger.kernel.org>,
	"dm-devel@redhat.com" <dm-devel@redhat.com>,
	Stefan Hajnoczi <stefanha@redhat.com>,
	Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: [PATCH v3] virtio_blk: add VIRTIO_BLK_F_LIFETIME feature support
Date: Tue, 13 Dec 2022 01:23:42 -0500	[thread overview]
Message-ID: <20221213001034-mutt-send-email-mst@kernel.org> (raw)
In-Reply-To: <ccb5388d-4976-31a3-0559-e94c14c90573@nvidia.com>

On Tue, Dec 13, 2022 at 04:58:47AM +0000, Chaitanya Kulkarni wrote:
> Michael,
> 
> On 12/7/22 12:28, Michael S. Tsirkin wrote:
> > On Wed, Dec 07, 2022 at 08:31:28AM -0800, Christoph Hellwig wrote:
> >> On Wed, Dec 07, 2022 at 05:21:48AM -0500, Michael S. Tsirkin wrote:
> >>> Christoph you acked the spec patch adding this to virtio blk:
> >>>
> >>> 	Still not a fan of the encoding, but at least it is properly documented
> >>> 	now:
> >>>
> >>> 	Acked-by: Christoph Hellwig <hch@lst.de>
> >>>
> >>> Did you change your mind then? Or do you prefer a different encoding for
> >>> the ioctl then? could you help sugesting what kind?
> >>
> >> Well, it is good enough documented for a spec.  I don't think it is
> >> a useful feature for Linux where virtio-blk is our minimum viable
> >> paravirtualized block driver.
> > 
> > No idea what this means, sorry.  Now that's in the spec I expect (some)
> > devices to implement it and if they do I see no reason not to expose the
> > data to userspace.
> > 
> 
> Even if any device implements is it can always use passthru commands.
> See below for more info...
> 
> > Alvaro could you pls explain the use-case? Christoph has doubts that
> > it's useful. Do you have a device implementing this?
> > 
> 
>  From what I know, virtio-blk should be kept minimal and should not
> add any storage specific IOCTLs or features that will end up loosing
> its generic nature.
> 
> The IOCTL we are trying to add is Flash storage specific which
> goes against the nature of generic storage and makes it non-generic.
> In case we approve this it will open the door for non-generic
> code/IOCTL in the virtio-blk and that needs to be avoided.

Wrt these fields that horse has bolted, it's in the spec.

> For any storage specific features or IOCTL (flash/HDD) it should
> be using it's own frontend such as virtio-scsi or e.g. nvme and
> not virtio-blk.
> 
> Hope this helps.
> 
> -ck

I don't understand what you are suggesting, sorry. It's a hardware
device. It can't just "switch to a different frontend".

-- 
MST