Re: [Qemu-arm] [PATCH v2 12/14] target/arm/kvm: scratch vcpu: Preserve input kvm_vcpu_init features

[Dave Martin]

On Thu, Jun 27, 2019 at 08:30:57AM +0100, Auger Eric wrote:
> Hi Drew,
> 
> On 6/21/19 6:34 PM, Andrew Jones wrote:
> > kvm_arm_create_scratch_host_vcpu() takes a struct kvm_vcpu_init
> > parameter. Rather than just using it as an output parameter to
> > pass back the preferred target, use it also as an input parameter,
> > allowing a caller to pass a selected target if they wish and to
> > also pass cpu features. If the caller doesn't want to select a
> > target they can pass -1 for the target which indicates they want
> > to use the preferred target and have it passed back like before.
> > 
> > Signed-off-by: Andrew Jones <address@hidden>
> > ---
> >  target/arm/kvm.c   | 20 +++++++++++++++-----
> >  target/arm/kvm32.c |  6 +++++-
> >  target/arm/kvm64.c |  6 +++++-
> >  3 files changed, 25 insertions(+), 7 deletions(-)
> > 
> > diff --git a/target/arm/kvm.c b/target/arm/kvm.c
> > index 60645a196d3d..66c0c198604a 100644
> > --- a/target/arm/kvm.c
> > +++ b/target/arm/kvm.c
> > @@ -64,7 +64,7 @@ bool kvm_arm_create_scratch_host_vcpu(const uint32_t 
> > *cpus_to_try,
> >                                        int *fdarray,
> >                                        struct kvm_vcpu_init *init)
> >  {
> > -    int ret, kvmfd = -1, vmfd = -1, cpufd = -1;
> > +    int ret = 0, kvmfd = -1, vmfd = -1, cpufd = -1;
> >  
> >      kvmfd = qemu_open("/dev/kvm", O_RDWR);
> >      if (kvmfd < 0) {
> > @@ -84,7 +84,14 @@ bool kvm_arm_create_scratch_host_vcpu(const uint32_t 
> > *cpus_to_try,
> >          goto finish;
> >      }
> >  
> > -    ret = ioctl(vmfd, KVM_ARM_PREFERRED_TARGET, init);
> > +    if (init->target == -1) {
> > +        struct kvm_vcpu_init preferred;
> > +
> > +        ret = ioctl(vmfd, KVM_ARM_PREFERRED_TARGET, &preferred);
> > +        if (!ret) {
> > +            init->target = preferred.target;
> wouldn't it be safe to copy the whole struct. Kernel code says:
>         /*
>          * For now, we don't return any features.
>          * In future, we might use features to return target
>          * specific features available for the preferred
>          * target type.
>          */

Marc or Christoffer should preferably comment on this.

I think the spirit of the ABI is that can use the whole return of
KVM_ARM_PREFERRED_TARGET as a reasonable template for KVM_VCPU_INIT
without it blowing up in your face.

I initially tried to report SVE as available through this route,
but we decided against it precisely because userspace might be doing
the above.

[...]

Cheers
---Dave