On Tue, 27 Jul 2021 19:10:27 +0100,
Will Deacon <will@xxxxxxxxxx> wrote:
>
> On Thu, Jul 15, 2021 at 05:31:44PM +0100, Marc Zyngier wrote:
> > We currently deal with a set of booleans for VM features,
> > while they could be better represented as set of flags
> > contained in an unsigned long, similarily to what we are
> > doing on the CPU side.
> >
> > Signed-off-by: Marc Zyngier <maz@xxxxxxxxxx>
> > ---
> > arch/arm64/include/asm/kvm_host.h | 12 +++++++-----
> > arch/arm64/kvm/arm.c | 5 +++--
> > arch/arm64/kvm/mmio.c | 3 ++-
> > 3 files changed, 12 insertions(+), 8 deletions(-)
> >
> > diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> > index 41911585ae0c..4add6c27251f 100644
> > --- a/arch/arm64/include/asm/kvm_host.h
> > +++ b/arch/arm64/include/asm/kvm_host.h
> > @@ -122,7 +122,10 @@ struct kvm_arch {
> > * should) opt in to this feature if KVM_CAP_ARM_NISV_TO_USER is
> > * supported.
> > */
> > - bool return_nisv_io_abort_to_user;
> > +#define KVM_ARCH_FLAG_RETURN_NISV_IO_ABORT_TO_USER 0
> > + /* Memory Tagging Extension enabled for the guest */
> > +#define KVM_ARCH_FLAG_MTE_ENABLED 1
> > + unsigned long flags;
>
> One downside of packing all these together is that updating 'flags' now
> requires an atomic rmw sequence (i.e. set_bit()). Then again, that's
> probably for the best anyway given that kvm_vm_ioctl_enable_cap() looks
> like it doesn't hold any locks.
That, and these operations are supposed to be extremely rare anyway.
>
> > /*
> > * VM-wide PMU filter, implemented as a bitmap and big enough for
> > @@ -133,9 +136,6 @@ struct kvm_arch {
> >
> > u8 pfr0_csv2;
> > u8 pfr0_csv3;
> > -
> > - /* Memory Tagging Extension enabled for the guest */
> > - bool mte_enabled;
> > };
> >
> > struct kvm_vcpu_fault_info {
> > @@ -777,7 +777,9 @@ bool kvm_arm_vcpu_is_finalized(struct kvm_vcpu *vcpu);
> > #define kvm_arm_vcpu_sve_finalized(vcpu) \
> > ((vcpu)->arch.flags & KVM_ARM64_VCPU_SVE_FINALIZED)
> >
> > -#define kvm_has_mte(kvm) (system_supports_mte() && (kvm)->arch.mte_enabled)
> > +#define kvm_has_mte(kvm) \
> > + (system_supports_mte() && \
> > + test_bit(KVM_ARCH_FLAG_MTE_ENABLED, &(kvm)->arch.flags))
>
> Not an issue with this patch, but I just noticed that the
> system_supports_mte() check is redundant here as we only allow the flag to
> be set if that's already the case.
It allows us to save a memory access if system_supports_mte() is false
(it is eventually implemented as a static key). On the other hand,
there is so much inlining due to it being a non-final cap that we
probably lose on that too...
M.
--
Without deviation from the norm, progress is not possible.
