On Thu, 21 Jan 2021 02:47:45 +0000,
Haibo Xu [off-list ref] wrote:

On Fri, 11 Dec 2020 at 00:04, Marc Zyngier [off-list ref] wrote:

quoted

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.

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