Subject: Re: [PATCH v5 34/69] KVM: arm64: nv: Configure HCR_EL2 for nested virtualization

From: Marc Zyngier <[email-masked]>
Date: Tue, 04 Jan 2022 09:39:59 +0000

On Tue, 04 Jan 2022 08:53:42 +0000,
Ganapatrao Kulkarni <[email-masked]> wrote:
> 
> 
> 
> On 30-11-2021 01:31 am, Marc Zyngier wrote:
> > From: Jintack Lim <[email-masked]>
> > 
> > We enable nested virtualization by setting the HCR NV and NV1 bit.
> > 
> > When the virtual E2H bit is set, we can support EL2 register accesses
> > via EL1 registers from the virtual EL2 by doing trap-and-emulate. A
> > better alternative, however, is to allow the virtual EL2 to access EL2
> > register states without trap. This can be easily achieved by not traping
> > EL1 registers since those registers already have EL2 register states.
> > 
> > Signed-off-by: Jintack Lim <[email-masked]>
> > Signed-off-by: Marc Zyngier <[email-masked]>
> > ---
> >   arch/arm64/include/asm/kvm_arm.h |  1 +
> >   arch/arm64/kvm/hyp/vhe/switch.c  | 38 +++++++++++++++++++++++++++++---
> >   2 files changed, 36 insertions(+), 3 deletions(-)
> > 
> > diff --git a/arch/arm64/include/asm/kvm_arm.h b/arch/arm64/include/asm/kvm_arm.h
> > index 68af5509e4b0..b8a0d410035b 100644
> > --- a/arch/arm64/include/asm/kvm_arm.h
> > +++ b/arch/arm64/include/asm/kvm_arm.h
> > @@ -87,6 +87,7 @@
> >   			 HCR_BSU_IS | HCR_FB | HCR_TACR | \
> >   			 HCR_AMO | HCR_SWIO | HCR_TIDCP | HCR_RW | HCR_TLOR | \
> >   			 HCR_FMO | HCR_IMO | HCR_PTW )
> > +#define HCR_GUEST_NV_FILTER_FLAGS (HCR_ATA | HCR_API | HCR_APK | HCR_RW)
> >   #define HCR_VIRT_EXCP_MASK (HCR_VSE | HCR_VI | HCR_VF)
> >   #define HCR_HOST_NVHE_FLAGS (HCR_RW | HCR_API | HCR_APK | HCR_ATA)
> >   #define HCR_HOST_NVHE_PROTECTED_FLAGS (HCR_HOST_NVHE_FLAGS | HCR_TSC)
> > diff --git a/arch/arm64/kvm/hyp/vhe/switch.c b/arch/arm64/kvm/hyp/vhe/switch.c
> > index 57f43e607819..da80c969e623 100644
> > --- a/arch/arm64/kvm/hyp/vhe/switch.c
> > +++ b/arch/arm64/kvm/hyp/vhe/switch.c
> > @@ -36,9 +36,41 @@ static void __activate_traps(struct kvm_vcpu *vcpu)
> >   	u64 hcr = vcpu->arch.hcr_el2;
> >   	u64 val;
> >   -	/* Trap VM sysreg accesses if an EL2 guest is not using
> > VHE. */
> > -	if (vcpu_mode_el2(vcpu) && !vcpu_el2_e2h_is_set(vcpu))
> > -		hcr |= HCR_TVM | HCR_TRVM;
> > +	if (is_hyp_ctxt(vcpu)) {
> > +		hcr |= HCR_NV;
> > +
> > +		if (!vcpu_el2_e2h_is_set(vcpu)) {
> > +			/*
> > +			 * For a guest hypervisor on v8.0, trap and emulate
> > +			 * the EL1 virtual memory control register accesses.
> > +			 */
> > +			hcr |= HCR_TVM | HCR_TRVM | HCR_NV1;
> > +		} else {
> > +			/*
> > +			 * For a guest hypervisor on v8.1 (VHE), allow to
> > +			 * access the EL1 virtual memory control registers
> > +			 * natively. These accesses are to access EL2 register
> > +			 * states.
> > +			 * Note that we still need to respect the virtual
> > +			 * HCR_EL2 state.
> > +			 */
> > +			u64 vhcr_el2 = __vcpu_sys_reg(vcpu, HCR_EL2);
> > +
> > +			vhcr_el2 &= ~HCR_GUEST_NV_FILTER_FLAGS;
> 
> Why HCR_RW is cleared here, May I know please?

Good question. That's clearly a leftover from an early rework. It
really doesn't matter, as we are merging the guest's configuration
into the host's, and the host already has HCR_EL2.RW set.

What HCR_GUEST_NV_FILTER_FLAGS should contain is only the bits we
don't want to deal with at this stage of the NV support. I'll fix that
for the next round.

Thanks,

	M.

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

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