Subject: Re: [PATCH v5 08/69] KVM: arm64: nv: Reset VCPU to EL2 registers if VCPU nested virt is set

From: Marc Zyngier <[email-masked]>
Date: Thu, 27 Jan 2022 12:42:42 +0000

On Fri, 07 Jan 2022 21:54:01 +0000,
Chase Conklin <[email-masked]> wrote:
> 
> Hi Marc,
> 
> On Mon Nov 29 15:00:49 EST 2021, Marc Zyngier <[email-masked]> wrote:
> > From: Christoffer Dall <christoffer.dall at arm.com>
> >
> > Reset the VCPU with PSTATE.M = EL2h when the nested virtualization
> > feature is enabled on the VCPU.
> >
> > Signed-off-by: Christoffer Dall <christoffer.dall at arm.com>
> > [maz: rework register reset not to use empty data structures]
> > Signed-off-by: Marc Zyngier <maz at kernel.org>
> > ---
> >  arch/arm64/kvm/reset.c | 10 ++++++++--
> >  1 file changed, 8 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/arm64/kvm/reset.c b/arch/arm64/kvm/reset.c
> > index 426bd7fbc3fd..38a7182819fb 100644
> > --- a/arch/arm64/kvm/reset.c
> > +++ b/arch/arm64/kvm/reset.c
> > @@ -27,6 +27,7 @@
> >  #include <asm/kvm_asm.h>
> >  #include <asm/kvm_emulate.h>
> >  #include <asm/kvm_mmu.h>
> > +#include <asm/kvm_nested.h>
> >  #include <asm/virt.h>
> >  
> >  /* Maximum phys_shift supported for any VM on this host */
> > @@ -38,6 +39,9 @@ static u32 kvm_ipa_limit;
> >  #define VCPU_RESET_PSTATE_EL1	(PSR_MODE_EL1h | PSR_A_BIT | PSR_I_BIT | \
> >  				 PSR_F_BIT | PSR_D_BIT)
> >  
> > +#define VCPU_RESET_PSTATE_EL2	(PSR_MODE_EL2h | PSR_A_BIT | PSR_I_BIT | \
> > +				 PSR_F_BIT | PSR_D_BIT)
> > +
> >  #define VCPU_RESET_PSTATE_SVC	(PSR_AA32_MODE_SVC | PSR_AA32_A_BIT | \
> >  				 PSR_AA32_I_BIT | PSR_AA32_F_BIT)
> >  
> > @@ -176,8 +180,8 @@ static bool vcpu_allowed_register_width(struct kvm_vcpu *vcpu)
> >  	if (!cpus_have_const_cap(ARM64_HAS_32BIT_EL1) && is32bit)
> >  		return false;
> >  
> > -	/* MTE is incompatible with AArch32 */
> > -	if (kvm_has_mte(vcpu->kvm) && is32bit)
> > +	/* MTE and NV are incompatible with AArch32 */
> > +	if ((kvm_has_mte(vcpu->kvm) || nested_virt_in_use(vcpu)) && is32bit)
> >  		return false;
> 
> Should something similar be done for SVE? I see from the ID register emulation
> that SVE is hidden from the guest but there isn't anything in
> kvm_vcpu_enable_sve() that checks if NV is in use. That means it's possible to
> have both nested_virt_in_use(vcpu) and vcpu_has_sve(vcpu) be true
> simultaneously. If that happens, the FPSIMD fixup can get confused
> 
> 	/*
> 	 * Don't handle SVE traps for non-SVE vcpus here. This
> 	 * includes NV guests for the time being.
> 	 */
> 	if (!sve_guest && (esr_ec != ESR_ELx_EC_FP_ASIMD ||
> 			   guest_hyp_fpsimd_traps_enabled(vcpu)))
> 		return false;
> 
> and incorrectly restore the wrong context instead of forwarding a
> FPSIMD trap to the guest hypervisor.

Yes, nice catch. I have added this to kvm_reset_vcpu() to prevent the
issue.

	if (nested_virt_in_use(vcpu) &&
	    vcpu_has_feature(vcpu, KVM_ARM_VCPU_SVE)) {
		ret = -EINVAL;
		goto out;
	}

I may also rename nested_virt_in_use() to vcpu_has_nv(), which would
fit the rest of the code a bit better.

Thanks,

	M.

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

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