RE: [RFC PATCH 0/6] KVM: arm64: Errata management for VM Live migration

[Cornelia Huck <cohuck@redhat.com>]

On Fri, Oct 11 2024, Shameerali Kolothum Thodi <shameerali.kolothum.thodi@huawei.com> wrote:

>> -----Original Message-----
>> From: Oliver Upton <oliver.upton@linux.dev>
>> Sent: Friday, October 11, 2024 4:11 PM
>> To: Marc Zyngier <maz@kernel.org>
>> Cc: Shameerali Kolothum Thodi
>> <shameerali.kolothum.thodi@huawei.com>; kvmarm@lists.linux.dev;
>> catalin.marinas@arm.com; will@kernel.org; mark.rutland@arm.com;
>> cohuck@redhat.com; eric.auger@redhat.com; yuzenghui
>> <yuzenghui@huawei.com>; Wangzhou (B) <wangzhou1@hisilicon.com>;
>> jiangkunkun <jiangkunkun@huawei.com>; Jonathan Cameron
>> <jonathan.cameron@huawei.com>; Anthony Jebson
>> <anthony.jebson@huawei.com>; linux-arm-kernel@lists.infradead.org;
>> Linuxarm <linuxarm@huawei.com>
>> Subject: Re: [RFC PATCH 0/6] KVM: arm64: Errata management for VM Live
>> migration
>> 
>> On Fri, Oct 11, 2024 at 12:43:28PM +0100, Marc Zyngier wrote:
>> > On Fri, 11 Oct 2024 11:57:10 +0100, Shameerali Kolothum Thodi
>> <shameerali.kolothum.thodi@huawei.com> wrote:
>> > > > >
>> > > > > Please take a look and let me know your thoughts.
>> > > >
>> > > > Having eyeballed this very superficially, I think we can do something
>> > > > simpler, and maybe more future-proof:
>> > >
>> > > Thanks Marc for taking a look and the quick feedback.
>> >
>> > No worries, that's the least I could do given that you put the effort
>> > implementing my silly ideas! ;-)
>> >
>> > > > - I don't think KVM should be concerned about the description of the
>> > > >   target CPUs. The hypercall you defined is the right thing to do,
>> > > >   but the VMM should completely handle it. That's an implementation
>> > > >   detail, but it would make things much simpler.
>> > >
>> > > Ok. So does that mean the hypercall will use some sort of shared
>> memory
>> > > to retrieve the list of target CPUs from VMM?
>> >
>> > Two possibilities:
>> >
>> > - either shared memory, in which case the hypercall would require the
>> >   guest to give an IPA and size for the VMM to write its stuff into
>> >   the guest memory,
>> >
>> > - or more simply return the data as an MIDR/REVIDR pair in registers,
>> >   the guest requesting an index, and getting an error when out of
>> >   range, leaving it with the freedom to organise the storage.
>> >
>> > The second option is a bit slower, but way simpler, and it only
>> > happens once per guest boot, so it would probably be my preferred
>> > option unless this is proved to be impractical.
>> 
>> Also worth noting there's existing UAPI [*] for allowing userspace to
>> register range(s) of hypercalls that it services directly. It's a bit
>> weird that we'd allow userspace to do stuff in KVM's own hypercall
>> range, but I don't think it really matters at this point since this is
>> all prototyping.
>> 
>> [*]: https://docs.kernel.org/virt/kvm/devices/vm.html#attribute-kvm-arm-
>> vm-smccc-filter-w-o
>
> Thanks. Yes and there are attempts to add that handling in Qemu[*] in the context
> of vCPU hotplug support(PSCI related ones though).  Will take a look.
>
> Thanks,
> Shameer
> [1] https://lore.kernel.org/qemu-devel/20241009033704.250287-1-salil.mehta@huawei.com/

Speaking of QEMU: Do you maybe already have some prototype code that
tries to do something with the setup here? (I don't think QEMU currently
mucks around with MIDR and friends when running with KVM; I wonder what
it should provide to the guest and if it should care to set something as
a base level that gives guests not using the hypercall a chance to work
properly.)