Re: [PATCH v3 00/16] KVM: arm64: GICv3 ITS emulation

[Peter Maydell <peter.maydell@linaro.org>]

On 14 March 2016 at 11:13, Andre Przywara <andre.przywara@arm.com> wrote:
> So I see two ways to fix this:
> 1.) we find a KVM specific way of letting userland save and restore the
> ITS tables directly
> 2.) we implement the BASER<n> registers, but still use our "cache" for
> normal operations. On demand we would serialize KVM's virtual ITS data
> structures and put them into the guest's memory, so they could be
> saved/restored from there.

I feel like we're rehashing a bunch of design choices we talked
through way back in the last-but-one Connect. I don't suppose
anybody wrote down our rationales from back then?

(In particular I forget whether we decided the ITS tables were
large enough to need to allow some sort of before-the-VM-stops
migration of the data, which would be relatively doable with
option 2 but painful under option 1.)

>> Only caveat there I think was that we had to decide on a storage format
>> in those memory regions, to allow QEMU to understand the state and to
>> ensure back/forwards compatibility between KVM versions.
>
> Do we need QEMU to actually understand this? Can't we just leave this
> all to the kernel and QEMU just passes on the data? That would still
> require some ABI stability between kernel versions in this respect, but
> it's less problematic than exposing the data format to userland at all.

This would preclude ever being able to migrate a VM from KVM to
TCG QEMU, which seems a shame. (That doesn't work right now, but
I'm a bit wary of shutting the door to it forever.)

thanks
-- PMM