On Mon, Nov 02, 2020 at 07:50:34PM +0100, Andrew Jones wrote:
> 张东旭 <xu910121@xxxxxxxx> reported a regression seen with CentOS
> when migrating from an old kernel to a new one. The problem was
> that QEMU rejected the migration since KVM_GET_REG_LIST reported
> a register was missing on the destination. Extra registers are OK
> on the destination, but not missing ones. The regression reproduces
> with upstream kernels when migrating from a 4.15 or later kernel,
> up to one with commit 73433762fcae ("KVM: arm64/sve: System register
> context switch and access support"), to a kernel that includes that
> commit, e.g. the latest mainline (5.10-rc2).
>
> The first patch of this series is the fix. The next two patches,
> which don't have any intended functional changes, allow ID_SANITISED
> to be used for registers that flip between exposing features and
> being RAZ, which allows some code to be removed.
Is it worth updating Documentation/virt/kvm/api.rst to clarify the
expected use during VM migrations, and the guarantees that are expected
to hold between migratable kernel versions? Currently the specification
is a mixture of "surely it's obvious" and "whatever makes QEMU work".
I guess that caught me out, but I'll let others judge whether other
people are likely to get similarly confused.
[...]
Cheers
---Dave
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