Re: Kernel patch cases qemu live migration failed.

[Peter Maydell]

On Thu, 15 Oct 2020 at 15:41, Andrew Jones <drjones@redhat.com> wrote:
> The reporter states neither the source nor destination hardware supports
> SVE. My guess is that what's happening is the reserved ID register
> ID_UNALLOCATED(4,4) was showing up in the KVM_GET_REG_LIST count on
> the old kernel, but the new kernel filters it out. Maybe it is a
> bug to filter it out of the count, as it's a reserved ID register and
> I suppose the other reserved ID registers are still showing up?

Yeah, RES0 ID registers should show up in the list, because otherwise
userspace has to annoyingly special case them when the architecture
eventually defines behaviour for them.

Dave's comment in the kernel commit message
# ID_AA64ZFR0_EL1 is RO-RAZ for MRS/MSR when SVE is disabled for the
# guest, but for compatibility with non-SVE aware KVM implementations
# the register should not be enumerated at all for KVM_GET_REG_LIST
# in this case.
seems wrong to me -- for compatibility the register should remain
present and behave as RAZ/WI if SVE is disabled in the guest,
the same way it was before the kernel/KVM knew about SVE at all.

thanks
-- PMM