Subject: Re: [PATCH v3 33/66] KVM: arm64: nv: Support multiple nested Stage-2 mmu structures

From: Marc Zyngier <[email-masked]>
Date: Tue, 16 Feb 2021 18:34:11 +0000

On Thu, 21 Jan 2021 02:59:13 +0000,
Haibo Xu <[email-masked]> wrote:
> 
> On Fri, 11 Dec 2020 at 00:04, Marc Zyngier <[email-masked]> wrote:

[...]

> > diff --git a/arch/arm64/kvm/nested.c b/arch/arm64/kvm/nested.c
> > index 9fb44bc7db3f..8e85d2ef24d9 100644
> > --- a/arch/arm64/kvm/nested.c
> > +++ b/arch/arm64/kvm/nested.c
> > @@ -19,12 +19,177 @@
> >  #include <linux/kvm.h>
> >  #include <linux/kvm_host.h>
> >
> > +#include <asm/kvm_arm.h>
> >  #include <asm/kvm_emulate.h>
> > +#include <asm/kvm_mmu.h>
> >  #include <asm/kvm_nested.h>
> >  #include <asm/sysreg.h>
> >
> >  #include "sys_regs.h"
> >
> > +void kvm_init_nested(struct kvm *kvm)
> > +{
> > +       kvm->arch.nested_mmus = NULL;
> > +       kvm->arch.nested_mmus_size = 0;
> > +}
> > +
> > +int kvm_vcpu_init_nested(struct kvm_vcpu *vcpu)
> > +{
> > +       struct kvm *kvm = vcpu->kvm;
> > +       struct kvm_s2_mmu *tmp;
> > +       int num_mmus;
> > +       int ret = -ENOMEM;
> > +
> > +       if (!test_bit(KVM_ARM_VCPU_HAS_EL2, vcpu->arch.features))
> > +               return 0;
> > +
> > +       if (!cpus_have_final_cap(ARM64_HAS_NESTED_VIRT))
> > +               return -EINVAL;
> 
> nit: returning a "not supported" kind of errno?

I think this is consistent with what we otherwise return when there is
a mismatch between requested features from userspace and host
capabilities (such as asking for a 32bit guest on a 64bit only CPU).

What seems to be missing though is a way to *advertise* the feature to
userspace as such:

diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
index 97d377265d8e..0225b81d3434 100644
--- a/arch/arm64/kvm/arm.c
+++ b/arch/arm64/kvm/arm.c
@@ -243,6 +243,9 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
 	case KVM_CAP_ARM_EL1_32BIT:
 		r = cpus_have_const_cap(ARM64_HAS_32BIT_EL1);
 		break;
+	case KVM_CAP_ARM_EL2:
+		r = cpus_have_const_cap(ARM64_HAS_NESTED_VIRT);
+		break;
 	case KVM_CAP_GUEST_DEBUG_HW_BPS:
 		r = get_num_brps();
 		break;
diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index 374c67875cdb..555a60ac00d8 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -1058,6 +1058,7 @@ struct kvm_ppc_resize_hpt {
 #define KVM_CAP_ENFORCE_PV_FEATURE_CPUID 190
 #define KVM_CAP_SYS_HYPERV_CPUID 191
 #define KVM_CAP_DIRTY_LOG_RING 192
+#define KVM_CAP_ARM_EL2 193
 
 #ifdef KVM_CAP_IRQ_ROUTING

Thanks,

	M.

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

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