Re: [PATCH v3 09/11] KVM: arm64: Introduce KVM_VM_TYPE_ARM_SW_PROTECTED machine type

[Fuad Tabba <tabba@google.com>]

Hi Quentin,

On Tue, 11 Feb 2025 at 16:12, Quentin Perret <qperret@google.com> wrote:
>
> Hi Fuad,
>
> On Tuesday 11 Feb 2025 at 12:11:25 (+0000), Fuad Tabba wrote:
> > diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> > index 117937a895da..f155d3781e08 100644
> > --- a/include/uapi/linux/kvm.h
> > +++ b/include/uapi/linux/kvm.h
> > @@ -652,6 +652,12 @@ struct kvm_enable_cap {
> >  #define KVM_VM_TYPE_ARM_IPA_SIZE_MASK        0xffULL
> >  #define KVM_VM_TYPE_ARM_IPA_SIZE(x)          \
> >       ((x) & KVM_VM_TYPE_ARM_IPA_SIZE_MASK)
> > +
> > +#define KVM_VM_TYPE_ARM_SW_PROTECTED (1UL << 9)
>
> FWIW, the downstream Android code has used bit 31 since forever
> for that.
>
> Although I very much believe that upstream should not care about the
> downstream mess in general, in this particular instance bit 9 really
> isn't superior in any way, and there's a bunch of existing userspace
> code that uses bit 31 today as we speak. It is very much Android's
> problem to update these userspace programs if we do go with bit 9
> upstream, but I don't really see how that would benefit upstream
> either.
>
> So, given that there is no maintenance cost for upstream to use bit 31
> instead of 9, I'd vote for using bit 31 and ease the landing with
> existing userspace code, unless folks are really opinionated with this
> stuff :)

My thinking is that this bit does _not_ mean pKVM. It means an
experimental software VM that is similar to the x86
KVM_X86_SW_PROTECTED_VM. Hence why I didn't choose bit 31.

From Documentation/virt/kvm/api.rst (for x86):

'''
Note, KVM_X86_SW_PROTECTED_VM is currently only for development and testing.
Do not use KVM_X86_SW_PROTECTED_VM for "real" VMs, and especially not in
production.  The behavior and effective ABI for software-protected VMs is
unstable.
'''

which is similar to the documentation I added here.

Cheers,
/fuad



> Thanks,
> Quentin
>
> > +#define KVM_VM_TYPE_MASK     (KVM_VM_TYPE_ARM_IPA_SIZE_MASK | \
> > +                              KVM_VM_TYPE_ARM_SW_PROTECTED)
> > +
> >  /*
> >   * ioctls for /dev/kvm fds:
> >   */
> > --
> > 2.48.1.502.g6dc24dfdaf-goog
> >