On Fri, 15 Feb 2019 09:58:57 +0000,
Andre Przywara <andre.przyw...@arm.com> wrote:
>
> On Wed, 30 Jan 2019 11:39:00 +0000
> Andre Przywara <andre.przyw...@arm.com> wrote:
>
> Peter, Marc, Christoffer,
>
> can we have an opinion on whether it's useful to introduce some
> common scheme for firmware workaround system registers (parts of
> KVM_REG_ARM_FW_REG(x)), which would allow checking them for
> compatibility between two kernels without specifically knowing about
> them?
> Dave suggested to introduce some kind of signed encoding in the 4
> LSBs for all those registers (including future ones), where 0 means
> UNKNOWN and greater values are better. So without knowing about the
> particular register, one could judge whether it's safe to migrate.
> I am just not sure how useful this is, given that QEMU seems to ask
> the receiving kernel about any sysreg, and doesn't particularly care
> about the meaning of those registers. And I am not sure we really
> want to introduce some kind of forward looking scheme in the kernel
> here, short of a working crystal ball. I think the kernel policy was
> always to be as strict as possible about those things.
I honestly don't understand how userspace can decide whether a given
configuration is migratable or not solely based on the value of such a
register. In my experience, the target system has a role to play, and
is the only place where we can find out about whether migration is
actually possible.
As you said, userspace doesn't interpret the data, nor should it. It
is only on the receiving end that compatibility is assessed and
whether some level of compatibility can be safely ensured.
So to sum it up, I don't believe in this approach as a general way of
describing the handling or errata.
Thanks,
M.
--
Jazz is not dead, it just smell funny.
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