Re: [PATCH 3/5] qmp: Added the helper stamp check.

[Daniel P . Berrangé]

On Sun, Feb 19, 2023 at 06:20:58PM +0200, Andrew Melnychenko wrote:
> Added a function to check the stamp in the helper.
> eBPF helper should have a special symbol that generates during the build.
> QEMU checks the helper and determines that it fits, so the helper
> will produce proper output.

I think this is quite limiting for in place upgrades.

Consider this scenario

 * Host has QEMU 8.1.0 installed
 * VM is running QEMU 8.1.0
 * QEMU 8.1.1 is released with a bug fix in the EBF program
 * Host is upgraded to QEMU 8.1.1
 * User attempts to hotplug a NIC to the running VM

IIUC this last step is going to fail because we'll be loading
the EBF program from 8.1.1 and so its hash is different from
that expected by the QEMU 8.1.0 that the pre-existing VM is
running.

If some changes to the EBF program are not going to be back
compatible from the POV of the QEMU process, should we instead
be versioning the EBF program. eg so new QEMU will ship both
the old and new versions of the EBF program. 


With regards,
Daniel
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