Re: Kernel patch cases qemu live migration failed.

[Peter Maydell]

On Mon, 19 Oct 2020 at 14:40, Andrew Jones <drjones@redhat.com> wrote:
>
> On Mon, Oct 19, 2020 at 12:43:33PM +0100, Peter Maydell wrote:
> > Well, ID regs are special in the architecture -- they always exist
> > and must RAZ/WI, even if they're not actually given any fields yet.
> > This is different from other "unused" parts of the system register
> > encoding space, which UNDEF.
>
> Table D12-2 confirms the register should be RAZ, as it says the register
> is "RO, but RAZ if SVE is not implemented". Does "RO" imply "WI", though?
> For the guest we inject an exception on writes, and for userspace we
> require the value to be preserved on write.

Sorry, I mis-spoke. They're RAZ, but not WI, just RO (which is to say
they'll UNDEF if you try to write to them).

> I think we should follow the spec, even for userspace access, and be RAZ
> for when the feature isn't implemented. As for writes, assuming the
> exception injection is what we want for the guest (not WI), then that's
> correct. For userspace, I think we should continue forcing preservation
> (which will force preservation of zero when it's RAZ).

Yes, that sounds right.

thanks
-- PMM