Re: [Qemu-devel] [PATCH 00/13] target/arm/kvm: enable SVE in guests

[Andrew Jones]

On Mon, May 13, 2019 at 10:52:06AM +0100, Peter Maydell wrote:
> On Sun, 12 May 2019 at 09:36, Andrew Jones <address@hidden> wrote:
> >
> > With the recent KVM guest SVE support pull request [1] KVM will be
> > ready for guests with SVE. This series provides the QEMU bits for
> > that enablement. The series starts with the bits needed for the KVM
> > SVE ioctls. Then it enables the arm 'max'cpu type, which with TCG
> > already supports SVE, to also support SVE when using KVM. Next
> > a new QMP query is added that allows users to ask which vector
> > lengths are supported by the host, allowing them to select a valid
> > set of vectors for the guest. In order to select those vectors a
> > new property 'sve-vls-map' is added to the 'max' cpu type, and then
> > also to the 'host' cpu type. The table below shows the resulting user
> > interfaces.
> >
> >    CPU type | accel | sve-max-vq | sve-vls-map
> >    -------------------------------------------
> >  1)     max | tcg   |  $MAX_VQ   |  $VLS_MAP
> >  2)     max | kvm   |  $MAX_VQ   |  $VLS_MAP
> >  3)    host | kvm   |  N/A       |  $VLS_MAP
> >
> > Where for (1) $MAX_VQ sets the maximum vq and smaller vqs are
> > all supported when $VLS_MAP is zero, or when the vqs are selected
> > in $VLS_MAP.
> >
> > (2) is the same as (1) except KVM has the final say on what
> > vqs are valid.
> >
> > (3) doesn't accept sve-max-vq because a guest that uses this
> > property without sve-vls-map cannot be safely migrated.
> 
> Is this "migrated between two hosts with the same host CPU
> type but with different kernel versions which expose different
> subsets of the host's permitted vector lengths" ?
>

That's one example. Also, I see attempting to make/use only migrate-safe
cpu properties for our primary KVM cpu type ('host') as a step in the
right direction to eventually making a more migratable KVM cpu type. But
I do see that (3) is a bit of an oxymoron considering the 'host' cpu
type isn't really migrate-safe unless the hosts are identical (which
should technically require the same host kernel too) anyway.

Thanks,
drew