Re: [Qemu-devel] [PATCH v4] virtio-blk: set correct config size for the host driver

From: Stefan Hajnoczi <stefanha@redhat.com>
To: Stefano Garzarella <sgarzare@redhat.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>,
	Stefan Hajnoczi <stefanha@gmail.com>,
	Changpeng Liu <changpeng.liu@intel.com>,
	qemu devel list <qemu-devel@nongnu.org>,
	Lukas Doktor <ldoktor@redhat.com>,
	"Dr. David Alan Gilbert" <dgilbert@redhat.com>,
	Greg Kurz <groug@kaod.org>
Subject: Re: [Qemu-devel] [PATCH v4] virtio-blk: set correct config size for the host driver
Date: Thu, 14 Feb 2019 11:37:17 +0800	[thread overview]
Message-ID: <20190214033717.GE21877@stefanha-x1.localdomain> (raw)
In-Reply-To: <CAGxU2F7Pn2N4fxr54jg4fUsNNT2Yo-OR10XHqfCLKv=BhQUCBQ@mail.gmail.com>

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On Wed, Feb 13, 2019 at 06:45:20PM +0100, Stefano Garzarella wrote:
> I'm not sure if using the feature_sizes array the migration works well.
> I mean if we have QEMU 3.1 with a single queue and we want to migrate to
> QEMU 4.0 with a single queue, the config_size could be different,
> because VIRTIO_NET_F_MQ is not set in the host_features.
> 
> Maybe we should initialize a config_size to the VIRTIO_BLK_CFG_SIZE macro
> defined by Changpeng and then use the feature_sizes arrays only for the
> new features (i.e.  discard, write_zeroes)
> 
> What do you think?

I agree.  The config size must not change for existing machine types.

Your code makes sense for future features but old features must use the
static config size.

Stefan

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