Re: [PATCH v2 0/4] vhost-user-blk: fix compatibility with older qemu versions

Stefan Hajnoczi <[email protected]> Thu, 26 Mar 2026 11:55:14 -0400

Newsgroups	dev.linux.lists.virtio-fs,org.nongnu.qemu-devel
Message-ID	<20260326155514.GF758087@fedora>

On Wed, Mar 11, 2026 at 01:47:24PM +0500, Alexandr Moshkov wrote:
> v1 -> v2:
> - reorganize commits: make refactor commits first, then core semantic change
> - add additional pre_save check for inflight migration possibility
> 
> ---
> 
> This is a small continuation of my series about inflight migration for vhost-user-blk.
> 
> This series is designed to solve the problem of compatibility with older versions of qemu, where this feature has not yet been introduced (for example, if we want to downgrade versions due to some problems).
> 
> In the current version for vhost-user-blk, this feature is enabled using the parameter and further migration of the inflight region will depend on whether the vhost-user has accepted the new protocol feature or not. This creates an inconvenient dependency, because there is no way to disable this feature without reconnecting to the backend.
> This series slightly changes the semantics of the introduced protocol feature VHOST_USER_PROTOCOL_F_GET_VRING_BASE_INFLIGHT. Enabling this feature adds a new parameter for GET_VRING_BASE message - skip_drain, which allows to control drain in-flight requests on the backend.
> 
> Thus, user can enable or disable inflight-migration param for vhost-user-blk to maintain compatibility with older versions of QEMU.

Can you describe what the user needs to do in more detail?

While the patch series introduces a way to tell the vhost-user back-end
whether or not to use inflight migration at runtime, I don't see a way
for the user to enable/disable inflight migration at runtime in QEMU. So
how does the user migrate back to an old QEMU if the current VM was
launched with inflight-migration=on?

Stefan

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