Re: [PATCH v12 23/29] KVM: x86: Implement gmem hook for determining max NPT mapping level

[Paolo Bonzini]

On 3/29/24 23:58, Michael Roth wrote:
> In the case of SEV-SNP, whether or not a 2MB page can be mapped via a
> 2MB mapping in the guest's nested page table depends on whether or not
> any subpages within the range have already been initialized as private
> in the RMP table. The existing mixed-attribute tracking in KVM is
> insufficient here, for instance:
>
>    - gmem allocates 2MB page
>    - guest issues PVALIDATE on 2MB page
>    - guest later converts a subpage to shared
>    - SNP host code issues PSMASH to split 2MB RMP mapping to 4K
>    - KVM MMU splits NPT mapping to 4K
>    - guest later converts that shared page back to private
>
> At this point there are no mixed attributes, and KVM would normally
> allow for 2MB NPT mappings again, but this is actually not allowed
> because the RMP table mappings are 4K and cannot be promoted on the
> hypervisor side, so the NPT mappings must still be limited to 4K to
> match this.
>
> Implement a kvm_x86_ops.gmem_validate_fault() hook for SEV that checks
> for this condition and adjusts the mapping level accordingly.
>
> Signed-off-by: Michael Roth <michael.roth@xxxxxxx>

Reviewed-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>

> ---
>   arch/x86/kvm/svm/sev.c | 32 ++++++++++++++++++++++++++++++++
>   arch/x86/kvm/svm/svm.c |  1 +
>   arch/x86/kvm/svm/svm.h |  7 +++++++
>   3 files changed, 40 insertions(+)
>
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index 87d621d013a4..31f6f4786503 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -4443,3 +4443,35 @@ void sev_gmem_invalidate(kvm_pfn_t start, kvm_pfn_t end)
>   		pfn += use_2m_update ? PTRS_PER_PMD : 1;
>   	}
>   }
> +
> +/*
> + * Re-check whether an #NPF for a private/gmem page can still be serviced, and
> + * adjust maximum mapping level if needed.
> + */
> +int sev_gmem_validate_fault(struct kvm *kvm, kvm_pfn_t pfn, gfn_t gfn, bool is_private,
> +			    u8 *max_level)
> +{
> +	int level, rc;
> +	bool assigned;
> +
> +	if (!sev_snp_guest(kvm))
> +		return 0;
> +
> +	rc = snp_lookup_rmpentry(pfn, &assigned, &level);
> +	if (rc) {
> +		pr_err_ratelimited("SEV: RMP entry not found: GFN %llx PFN %llx level %d error %d\n",
> +				   gfn, pfn, level, rc);
> +		return -ENOENT;
> +	}
> +
> +	if (!assigned) {
> +		pr_err_ratelimited("SEV: RMP entry is not assigned: GFN %llx PFN %llx level %d\n",
> +				   gfn, pfn, level);
> +		return -EINVAL;
> +	}
> +
> +	if (level < *max_level)
> +		*max_level = level;
> +
> +	return 0;
> +}
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index b456906f2670..298b4ce77a5f 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -5081,6 +5081,7 @@ static struct kvm_x86_ops svm_x86_ops __initdata = {
>   
>   	.gmem_prepare = sev_gmem_prepare,
>   	.gmem_invalidate = sev_gmem_invalidate,
> +	.gmem_validate_fault = sev_gmem_validate_fault,
>   };
>   
>   /*
> diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
> index 3f1f6d3d3ade..746f819a6de4 100644
> --- a/arch/x86/kvm/svm/svm.h
> +++ b/arch/x86/kvm/svm/svm.h
> @@ -732,6 +732,8 @@ void sev_vcpu_unblocking(struct kvm_vcpu *vcpu);
>   void sev_snp_init_protected_guest_state(struct kvm_vcpu *vcpu);
>   int sev_gmem_prepare(struct kvm *kvm, kvm_pfn_t pfn, gfn_t gfn, int max_order);
>   void sev_gmem_invalidate(kvm_pfn_t start, kvm_pfn_t end);
> +int sev_gmem_validate_fault(struct kvm *kvm, kvm_pfn_t pfn, gfn_t gfn, bool is_private,
> +			    u8 *max_level);
>   #else
>   static inline struct page *snp_safe_alloc_page(struct kvm_vcpu *vcpu) {
>   	return alloc_page(GFP_KERNEL_ACCOUNT | __GFP_ZERO);
> @@ -753,6 +755,11 @@ static inline int sev_gmem_prepare(struct kvm *kvm, kvm_pfn_t pfn, gfn_t gfn, in
>   	return 0;
>   }
>   static inline void sev_gmem_invalidate(kvm_pfn_t start, kvm_pfn_t end) {}
> +static inline int sev_gmem_validate_fault(struct kvm *kvm, kvm_pfn_t pfn, gfn_t gfn,
> +					  bool is_private, u8 *max_level)
> +{
> +	return 0;
> +}
>   
>   #endif
>