On Tue, Aug 10, 2021 at 2:35 AM Marc Zyngier <maz@xxxxxxxxxx> wrote:
>
> On Wed, 04 Aug 2021 09:58:11 +0100,
> Oliver Upton <oupton@xxxxxxxxxx> 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@xxxxxxxxxx>
> > ---
> > 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
