Re: [PATCH 1/5] KVM: arm64: Correctly honor the presence of FEAT_TCRX

[Marc Zyngier <maz@kernel.org>]

On Tue, 25 Jun 2024 15:37:34 +0100,
Joey Gouly <joey.gouly@arm.com> wrote:
> 
> On Tue, Jun 25, 2024 at 02:00:37PM +0100, Marc Zyngier wrote:
> > We currently blindly enable TCR2_EL1 use in a guest, irrespective
> > of the feature set. This is obviously wrong, and we should actually
> > honor the guest configuration and handle the possible trap resulting
> > from the guest being buggy.
> > 
> > Signed-off-by: Marc Zyngier <maz@kernel.org>
> > ---
> >  arch/arm64/include/asm/kvm_arm.h | 2 +-
> >  arch/arm64/kvm/sys_regs.c        | 9 +++++++++
> >  2 files changed, 10 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/arm64/include/asm/kvm_arm.h b/arch/arm64/include/asm/kvm_arm.h
> > index b2adc2c6c82a5..e6682a3ace5af 100644
> > --- a/arch/arm64/include/asm/kvm_arm.h
> > +++ b/arch/arm64/include/asm/kvm_arm.h
> > @@ -102,7 +102,7 @@
> >  #define HCR_HOST_NVHE_PROTECTED_FLAGS (HCR_HOST_NVHE_FLAGS | HCR_TSC)
> >  #define HCR_HOST_VHE_FLAGS (HCR_RW | HCR_TGE | HCR_E2H)
> >  
> > -#define HCRX_GUEST_FLAGS (HCRX_EL2_SMPME | HCRX_EL2_TCR2En)
> > +#define HCRX_GUEST_FLAGS (HCRX_EL2_SMPME)
> >  #define HCRX_HOST_FLAGS (HCRX_EL2_MSCEn | HCRX_EL2_TCR2En | HCRX_EL2_EnFPM)
> >  
> >  /* TCR_EL2 Registers bits */
> > diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> > index 22b45a15d0688..71996d36f3751 100644
> > --- a/arch/arm64/kvm/sys_regs.c
> > +++ b/arch/arm64/kvm/sys_regs.c
> > @@ -383,6 +383,12 @@ static bool access_vm_reg(struct kvm_vcpu *vcpu,
> >  	bool was_enabled = vcpu_has_cache_enabled(vcpu);
> >  	u64 val, mask, shift;
> >  
> > +	if (reg_to_encoding(r) == SYS_TCR2_EL1 &&
> > +	    !kvm_has_feat(vcpu->kvm, ID_AA64MMFR3_EL1, TCRX, IMP)) {
> > +		kvm_inject_undefined(vcpu);
> > +		return false;
> > +	}
> > +
> 
> If we need to start doing this with more vm(sa) registers, it might make sense
> to think of a way to do this without putting a big if/else in here.  For now
> this is seems fine.

One possible solution would be to mimic the FGU behaviour and have a
shadow version of HCRX_EL2 that only indicates the trap routing code
that something trapped through that bit needs to UNDEF.

And yes, I'd expect we'll see a whole lot of new VMSA registers going
the same way.

> 
> >  	BUG_ON(!p->is_write);
> >  
> >  	get_access_mask(r, &mask, &shift);
> > @@ -4060,6 +4066,9 @@ void kvm_init_sysreg(struct kvm_vcpu *vcpu)
> >  
> >  		if (kvm_has_feat(kvm, ID_AA64ISAR2_EL1, MOPS, IMP))
> >  			vcpu->arch.hcrx_el2 |= (HCRX_EL2_MSCEn | HCRX_EL2_MCE2);
> > +
> > +		if (kvm_has_feat(kvm, ID_AA64MMFR3_EL1, TCRX, IMP))
> > +			vcpu->arch.hcrx_el2 |= HCRX_EL2_TCR2En;
> >  	}
> >  
> >  	if (test_bit(KVM_ARCH_FLAG_FGU_INITIALIZED, &kvm->arch.flags))
> 
> Reviewed-by: Joey Gouly <joey.gouly@arm.com>

Thanks!

	M.

-- 
Without deviation from the norm, progress is not possible.