Re: [PATCH v6 13/21] KVM: arm64: Allow userspace to configure a vCPU's virtual offset

[Oliver Upton <oupton@google.com>]

On Tue, Aug 10, 2021 at 2:35 AM Marc Zyngier <maz@kernel.org> wrote:
>
> On Wed, 04 Aug 2021 09:58:11 +0100,
> Oliver Upton <oupton@google.com> wrote:
> >
> > Allow userspace to access the guest's virtual counter-timer offset
> > through the ONE_REG interface. The value read or written is defined to
> > be an offset from the guest's physical counter-timer. Add some
> > documentation to clarify how a VMM should use this and the existing
> > CNTVCT_EL0.
> >
> > Signed-off-by: Oliver Upton <oupton@google.com>
> > ---
> >  Documentation/virt/kvm/api.rst    | 10 ++++++++++
> >  arch/arm64/include/uapi/asm/kvm.h |  1 +
> >  arch/arm64/kvm/arch_timer.c       | 11 +++++++++++
> >  arch/arm64/kvm/guest.c            |  6 +++++-
> >  include/kvm/arm_arch_timer.h      |  1 +
> >  5 files changed, 28 insertions(+), 1 deletion(-)
> >
> > diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> > index 8d4a3471ad9e..28a65dc89985 100644
> > --- a/Documentation/virt/kvm/api.rst
> > +++ b/Documentation/virt/kvm/api.rst
> > @@ -2487,6 +2487,16 @@ arm64 system registers have the following id bit patterns::
> >       derived from the register encoding for CNTV_CVAL_EL0.  As this is
> >       API, it must remain this way.
> >
> > +.. warning::
> > +
> > +     The value of KVM_REG_ARM_TIMER_OFFSET is defined as an offset from
> > +     the guest's view of the physical counter-timer.
> > +
> > +     Userspace should use either KVM_REG_ARM_TIMER_OFFSET or
> > +     KVM_REG_ARM_TIMER_CVAL to pause and resume a guest's virtual
>
> You probably mean KVM_REG_ARM_TIMER_CNT here, despite the broken
> encoding.

Indeed I do!

>
> > +     counter-timer. Mixed use of these registers could result in an
> > +     unpredictable guest counter value.
> > +
> >  arm64 firmware pseudo-registers have the following bit pattern::
> >
> >    0x6030 0000 0014 <regno:16>
> > diff --git a/arch/arm64/include/uapi/asm/kvm.h b/arch/arm64/include/uapi/asm/kvm.h
> > index b3edde68bc3e..949a31bc10f0 100644
> > --- a/arch/arm64/include/uapi/asm/kvm.h
> > +++ b/arch/arm64/include/uapi/asm/kvm.h
> > @@ -255,6 +255,7 @@ struct kvm_arm_copy_mte_tags {
> >  #define KVM_REG_ARM_TIMER_CTL                ARM64_SYS_REG(3, 3, 14, 3, 1)
> >  #define KVM_REG_ARM_TIMER_CVAL               ARM64_SYS_REG(3, 3, 14, 0, 2)
> >  #define KVM_REG_ARM_TIMER_CNT                ARM64_SYS_REG(3, 3, 14, 3, 2)
> > +#define KVM_REG_ARM_TIMER_OFFSET     ARM64_SYS_REG(3, 4, 14, 0, 3)
>
> I don't think we can use the encoding for CNTPOFF_EL2 here, as it will
> eventually clash with a NV guest using the same feature for its own
> purpose. We don't want this offset to overlap with any of the existing
> features.
>
> I actually liked your previous proposal of controlling the physical
> offset via a device property, as it clearly indicated that you were
> dealing with non-architectural state.

That's actually exactly what I did here :) That said, the macro name
is horribly obfuscated from CNTVOFF_EL2. I did this for the sake of
symmetry with other virtual counter-timer registers above, though this
may warrant special casing given the fact that we have a similarly
named device attribute to handle the physical offset.

--
Thanks,
Oliver