Re: [PATCH 4/4] KVM: arm64: Always return generic v8 as the preferred target

[Oliver Upton <oliver.upton@linux.dev>]

On Mon, Jul 10, 2023 at 11:13:34PM +0800, Zenghui Yu wrote:
> Hi Oliver,
> 
> On 2023/6/24 3:42, Oliver Upton wrote:
> > Userspace selecting an implementation-specific vCPU target has been
> > completely useless for a very long time. Let's go whole hog and start
> > returning the generic v8 target across all implementations as the
> > preferred target.
> > 
> > Uphold the pre-existing behavior by tolerating either the generic target
> > or an implementation-specific target if the vCPU happens to be running
> > on one of the lucky few parts.
> > 
> > Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
> 
> I love the idea.
> 
> Acked-by: Zenghui Yu <yuzenghui@huawei.com>

Thanks!

> > ---
> >  arch/arm64/kvm/arm.c   | 3 ++-
> >  arch/arm64/kvm/guest.c | 4 +---
> >  2 files changed, 3 insertions(+), 4 deletions(-)
> > 
> > diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> > index 3fa63fdbbf34..5a3b8b2e779b 100644
> > --- a/arch/arm64/kvm/arm.c
> > +++ b/arch/arm64/kvm/arm.c
> > @@ -1250,7 +1250,8 @@ static int kvm_vcpu_set_target(struct kvm_vcpu *vcpu,
> >  {
> >  	int ret;
> > -	if (init->target != kvm_target_cpu())
> > +	if (init->target != KVM_ARM_TARGET_GENERIC_V8 &&
> > +	    init->target != kvm_target_cpu())
> >  		return -EINVAL;
> 
> I can see that user-space can now select GENERIC_V8 or CORTEX_A53 (for
> example) between two KVM_ARM_VCPU_INIT calls on an A53 host, which was
> forbidden before this change. I don't think it's a big deal though.

Agreed, I thought the slight relaxation on invariance was OK. If needed,
we can always change the rules for a new target value, but I doubt we will
need to do that anytime soon.

-- 
Thanks,
Oliver