Re: [PATCH v1] target/i386: kvm: Block migration when enfore_cpuid is set to false

["Daniel P. Berrangé" <berrange@redhat.com>]

On Wed, Jul 03, 2024 at 10:49:12PM +0800, Wei Wang wrote:
> When enforce_cpuid is set to false, the guest is launched with a filtered
> set of features, meaning that unsupported features by the host are removed
> from the guest's vCPU model. This could cause issues for live migration.
> For example, a guest on the source is running with features A and B. If
> the destination host does not support feature B, the stub guest can still
> be launched on the destination with feature A only if enforce_cpuid=false.
> Live migration can start in this case, though it may fail later when the
> states of feature B are put to the destination side. This failure occurs
> in the late stage (i.e., stop&copy phase) of the migration flow, where the
> source guest has already been paused. Tests show that in such cases the
> source guest does not recover, and the destination is unable to resume to
> run.
> 
> Make "enfore_cpuid=true" a hard requirement for a guest to be migratable,
> and change the default value of "enforce_cpuid" to true, making the guest
> vCPUs migratable by default. If the destination stub guest has inconsistent
> CPUIDs (i.e., destination host cannot support the features defined by the
> guest's vCPU model), it fails to boot (with enfore_cpuid=true by default),
> thereby preventing migration from occuring. If enfore_cpuid=false is
> explicitly added for the guest, the guest is deemed as non-migratable
> (via the migration blocker), so the above issue won't occur as the guest
> won't be migrated.

Blocking migration when enforce=false is making an assumption
that users of that setting are inherantly broken. This is NOT
the case if the user/app has already validated compatibility in
some manner outside QEMU. Blocking migration in this case will
break valid working use cases.

IMHO this patch doesn't need to exist. If users of QEMU want
strong protection they can already opt-in to that with enforce=true.

With regards,
Daniel
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