Re: Kernel patch cases qemu live migration failed.

[Andrew Jones]

On Thu, Oct 15, 2020 at 03:57:02PM +0100, Peter Maydell wrote:
> 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.

Yup, I agree with you and I'll try writing a patch for this.

Thanks,
drew