Re: [PATCH v3 63/66] KVM: arm64: nv: Allocate VNCR page when required

Marc Zyngier
2021-02-16T19:04:00Z

On Thu, 21 Jan 2021 02:47:45 +0000,
Haibo Xu <haibo.xu@linaro.org> wrote:
>
> On Fri, 11 Dec 2020 at 00:04, Marc Zyngier <maz@kernel.org> wrote:
> >
> > If running a NV guest on an ARMv8.4-NV capable system, let's
> > allocate an additional page that will be used by the hypervisor
> > to fulfill system register accesses.
> >
> > Signed-off-by: Marc Zyngier <maz@kernel.org>
> > ---
> > arch/arm64/include/asm/kvm_host.h | 3 ++-
> > arch/arm64/kvm/nested.c | 8 ++++++++
> > arch/arm64/kvm/reset.c | 1 +
> > 3 files changed, 11 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> > index 78630bd5124d..dada0678c28e 100644
> > --- a/arch/arm64/include/asm/kvm_host.h
> > +++ b/arch/arm64/include/asm/kvm_host.h
> > @@ -523,7 +523,8 @@ struct kvm_vcpu_arch {
> > */
> > static inline u64 *__ctxt_sys_reg(const struct kvm_cpu_context *ctxt, int r)
> > {
> > - if (unlikely(r >= __VNCR_START__ && ctxt->vncr_array))
> > + if (unlikely(cpus_have_final_cap(ARM64_HAS_ENHANCED_NESTED_VIRT) &&
> > + r >= __VNCR_START__ && ctxt->vncr_array))
> > return &ctxt->vncr_array[r - __VNCR_START__];
> >
> > return (u64 *)&ctxt->sys_regs[r];
> > diff --git a/arch/arm64/kvm/nested.c b/arch/arm64/kvm/nested.c
> > index eef8f9873814..88147ec99755 100644
> > --- a/arch/arm64/kvm/nested.c
> > +++ b/arch/arm64/kvm/nested.c
> > @@ -47,6 +47,12 @@ int kvm_vcpu_init_nested(struct kvm_vcpu *vcpu)
> > if (!cpus_have_final_cap(ARM64_HAS_NESTED_VIRT))
> > return -EINVAL;
> >
> > + if (cpus_have_final_cap(ARM64_HAS_ENHANCED_NESTED_VIRT)) {
> > + vcpu->arch.ctxt.vncr_array = (u64 *)__get_free_page(GFP_KERNEL | __GFP_ZERO);
> > + if (!vcpu->arch.ctxt.vncr_array)
> > + return -ENOMEM;
> > + }
> > +
>
> If KVM_ARM_VCPU_INIT was called multiple times, the above codes
> would try to allocate a new page without free-ing the previous
> one. Besides that, the following kvm_free_stage2_pgd() call would

I assume you mean kvm_init_stage2_mmu() here.

> fail in the second call with the error message "kvm_arch already
> initialized?". I think a possible fix is to add a new flag to
> indicate whether the NV related meta data have been initialized, and
> only initialize them for the first call.

Good catch. But I think we have all the data we need at this stage to
avoid this issue:

diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
index abb0669bdd4c..baff7373863f 100644
--- a/arch/arm64/kvm/mmu.c
+++ b/arch/arm64/kvm/mmu.c
@@ -390,7 +390,20 @@ int kvm_init_stage2_mmu(struct kvm *kvm, struct kvm_s2_mmu *mmu)
int cpu, err;
struct kvm_pgtable *pgt;

+ /*
+ * If we already have our page tables in place, and that the
+ * MMU context is the canonical one, we have a bug somewhere,
+ * as this is only supposed to ever happen once per VM.
+ *
+ * Otherwise, we're building nested page tables, and that's
+ * probably because userspace called KVM_ARM_VCPU_INIT more
+ * than once on the same vcpu. Since that's actually legal,
+ * don't kick a fuss and leave gracefully.
+ */
if (mmu->pgt != NULL) {
+ if (&kvm->arch.mmu != mmu)
+ return 0;
+
kvm_err("kvm_arch already initialized?\n");
return -EINVAL;
}
diff --git a/arch/arm64/kvm/nested.c b/arch/arm64/kvm/nested.c
index 88147ec99755..3b21ea57fbce 100644
--- a/arch/arm64/kvm/nested.c
+++ b/arch/arm64/kvm/nested.c
@@ -48,7 +48,9 @@ int kvm_vcpu_init_nested(struct kvm_vcpu *vcpu)
return -EINVAL;

if (cpus_have_final_cap(ARM64_HAS_ENHANCED_NESTED_VIRT)) {
- vcpu->arch.ctxt.vncr_array = (u64 *)__get_free_page(GFP_KERNEL | __GFP_ZERO);
+ if (!vcpu->arch.ctxt.vncr_array)
+ vcpu->arch.ctxt.vncr_array = (u64 *)__get_free_page(GFP_KERNEL | __GFP_ZERO);
+
if (!vcpu->arch.ctxt.vncr_array)
return -ENOMEM;
}

Thanks,

M.

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