Re: [PATCH v2 21/25] KVM: SEV: Make AVIC backing, VMSA and VMCB memory allocation SNP safe

[Paolo Bonzini <pbonzini@redhat.com>]

On Fri, Jan 26, 2024 at 5:45 AM Michael Roth <michael.roth@amd.com> wrote:
>
> From: Brijesh Singh <brijesh.singh@amd.com>
>
> Implement a workaround for an SNP erratum where the CPU will incorrectly
> signal an RMP violation #PF if a hugepage (2MB or 1GB) collides with the
> RMP entry of a VMCB, VMSA or AVIC backing page.
>
> When SEV-SNP is globally enabled, the CPU marks the VMCB, VMSA, and AVIC
> backing pages as "in-use" via a reserved bit in the corresponding RMP
> entry after a successful VMRUN. This is done for _all_ VMs, not just
> SNP-Active VMs.
>
> If the hypervisor accesses an in-use page through a writable
> translation, the CPU will throw an RMP violation #PF. On early SNP
> hardware, if an in-use page is 2MB-aligned and software accesses any
> part of the associated 2MB region with a hugepage, the CPU will
> incorrectly treat the entire 2MB region as in-use and signal a an RMP
> violation #PF.
>
> To avoid this, the recommendation is to not use a 2MB-aligned page for
> the VMCB, VMSA or AVIC pages. Add a generic allocator that will ensure
> that the page returned is not 2MB-aligned and is safe to be used when
> SEV-SNP is enabled. Also implement similar handling for the VMCB/VMSA
> pages of nested guests.
>
> Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
> Co-developed-by: Marc Orr <marcorr@google.com>
> Signed-off-by: Marc Orr <marcorr@google.com>
> Reported-by: Alper Gun <alpergun@google.com> # for nested VMSA case
> Co-developed-by: Ashish Kalra <ashish.kalra@amd.com>
> Signed-off-by: Ashish Kalra <ashish.kalra@amd.com>
> Acked-by: Vlastimil Babka <vbabka@suse.cz>
> [mdr: squash in nested guest handling from Ashish, commit msg fixups]
> Signed-off-by: Michael Roth <michael.roth@amd.com>

Acked-by: Paolo Bonzini <pbonzini@redhat.com>


> ---
>  arch/x86/include/asm/kvm-x86-ops.h |  1 +
>  arch/x86/include/asm/kvm_host.h    |  1 +
>  arch/x86/kvm/lapic.c               |  5 ++++-
>  arch/x86/kvm/svm/nested.c          |  2 +-
>  arch/x86/kvm/svm/sev.c             | 32 ++++++++++++++++++++++++++++++
>  arch/x86/kvm/svm/svm.c             | 17 +++++++++++++---
>  arch/x86/kvm/svm/svm.h             |  1 +
>  7 files changed, 54 insertions(+), 5 deletions(-)
>
> diff --git a/arch/x86/include/asm/kvm-x86-ops.h b/arch/x86/include/asm/kvm-x86-ops.h
> index 378ed944b849..ab24ce207988 100644
> --- a/arch/x86/include/asm/kvm-x86-ops.h
> +++ b/arch/x86/include/asm/kvm-x86-ops.h
> @@ -138,6 +138,7 @@ KVM_X86_OP(complete_emulated_msr)
>  KVM_X86_OP(vcpu_deliver_sipi_vector)
>  KVM_X86_OP_OPTIONAL_RET0(vcpu_get_apicv_inhibit_reasons);
>  KVM_X86_OP_OPTIONAL(get_untagged_addr)
> +KVM_X86_OP_OPTIONAL(alloc_apic_backing_page)
>
>  #undef KVM_X86_OP
>  #undef KVM_X86_OP_OPTIONAL
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index b5b2d0fde579..5c12af29fd9b 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -1794,6 +1794,7 @@ struct kvm_x86_ops {
>         unsigned long (*vcpu_get_apicv_inhibit_reasons)(struct kvm_vcpu *vcpu);
>
>         gva_t (*get_untagged_addr)(struct kvm_vcpu *vcpu, gva_t gva, unsigned int flags);
> +       void *(*alloc_apic_backing_page)(struct kvm_vcpu *vcpu);
>  };
>
>  struct kvm_x86_nested_ops {
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index 3242f3da2457..1edf93ee3395 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -2815,7 +2815,10 @@ int kvm_create_lapic(struct kvm_vcpu *vcpu, int timer_advance_ns)
>
>         vcpu->arch.apic = apic;
>
> -       apic->regs = (void *)get_zeroed_page(GFP_KERNEL_ACCOUNT);
> +       if (kvm_x86_ops.alloc_apic_backing_page)
> +               apic->regs = static_call(kvm_x86_alloc_apic_backing_page)(vcpu);
> +       else
> +               apic->regs = (void *)get_zeroed_page(GFP_KERNEL_ACCOUNT);
>         if (!apic->regs) {
>                 printk(KERN_ERR "malloc apic regs error for vcpu %x\n",
>                        vcpu->vcpu_id);
> diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
> index dee62362a360..55b9a6d96bcf 100644
> --- a/arch/x86/kvm/svm/nested.c
> +++ b/arch/x86/kvm/svm/nested.c
> @@ -1181,7 +1181,7 @@ int svm_allocate_nested(struct vcpu_svm *svm)
>         if (svm->nested.initialized)
>                 return 0;
>
> -       vmcb02_page = alloc_page(GFP_KERNEL_ACCOUNT | __GFP_ZERO);
> +       vmcb02_page = snp_safe_alloc_page(&svm->vcpu);
>         if (!vmcb02_page)
>                 return -ENOMEM;
>         svm->nested.vmcb02.ptr = page_address(vmcb02_page);
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index 564091f386f7..f99435b6648f 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -3163,3 +3163,35 @@ void sev_vcpu_deliver_sipi_vector(struct kvm_vcpu *vcpu, u8 vector)
>
>         ghcb_set_sw_exit_info_2(svm->sev_es.ghcb, 1);
>  }
> +
> +struct page *snp_safe_alloc_page(struct kvm_vcpu *vcpu)
> +{
> +       unsigned long pfn;
> +       struct page *p;
> +
> +       if (!cpu_feature_enabled(X86_FEATURE_SEV_SNP))
> +               return alloc_page(GFP_KERNEL_ACCOUNT | __GFP_ZERO);
> +
> +       /*
> +        * Allocate an SNP-safe page to workaround the SNP erratum where
> +        * the CPU will incorrectly signal an RMP violation #PF if a
> +        * hugepage (2MB or 1GB) collides with the RMP entry of a
> +        * 2MB-aligned VMCB, VMSA, or AVIC backing page.
> +        *
> +        * Allocate one extra page, choose a page which is not
> +        * 2MB-aligned, and free the other.
> +        */
> +       p = alloc_pages(GFP_KERNEL_ACCOUNT | __GFP_ZERO, 1);
> +       if (!p)
> +               return NULL;
> +
> +       split_page(p, 1);
> +
> +       pfn = page_to_pfn(p);
> +       if (IS_ALIGNED(pfn, PTRS_PER_PMD))
> +               __free_page(p++);
> +       else
> +               __free_page(p + 1);
> +
> +       return p;
> +}
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index 61f2bdc9f4f8..272d5ed37ce7 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -703,7 +703,7 @@ static int svm_cpu_init(int cpu)
>         int ret = -ENOMEM;
>
>         memset(sd, 0, sizeof(struct svm_cpu_data));
> -       sd->save_area = alloc_page(GFP_KERNEL | __GFP_ZERO);
> +       sd->save_area = snp_safe_alloc_page(NULL);
>         if (!sd->save_area)
>                 return ret;
>
> @@ -1421,7 +1421,7 @@ static int svm_vcpu_create(struct kvm_vcpu *vcpu)
>         svm = to_svm(vcpu);
>
>         err = -ENOMEM;
> -       vmcb01_page = alloc_page(GFP_KERNEL_ACCOUNT | __GFP_ZERO);
> +       vmcb01_page = snp_safe_alloc_page(vcpu);
>         if (!vmcb01_page)
>                 goto out;
>
> @@ -1430,7 +1430,7 @@ static int svm_vcpu_create(struct kvm_vcpu *vcpu)
>                  * SEV-ES guests require a separate VMSA page used to contain
>                  * the encrypted register state of the guest.
>                  */
> -               vmsa_page = alloc_page(GFP_KERNEL_ACCOUNT | __GFP_ZERO);
> +               vmsa_page = snp_safe_alloc_page(vcpu);
>                 if (!vmsa_page)
>                         goto error_free_vmcb_page;
>
> @@ -4900,6 +4900,16 @@ static int svm_vm_init(struct kvm *kvm)
>         return 0;
>  }
>
> +static void *svm_alloc_apic_backing_page(struct kvm_vcpu *vcpu)
> +{
> +       struct page *page = snp_safe_alloc_page(vcpu);
> +
> +       if (!page)
> +               return NULL;
> +
> +       return page_address(page);
> +}
> +
>  static struct kvm_x86_ops svm_x86_ops __initdata = {
>         .name = KBUILD_MODNAME,
>
> @@ -5031,6 +5041,7 @@ static struct kvm_x86_ops svm_x86_ops __initdata = {
>
>         .vcpu_deliver_sipi_vector = svm_vcpu_deliver_sipi_vector,
>         .vcpu_get_apicv_inhibit_reasons = avic_vcpu_get_apicv_inhibit_reasons,
> +       .alloc_apic_backing_page = svm_alloc_apic_backing_page,
>  };
>
>  /*
> diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
> index 8ef95139cd24..7f1fbd874c45 100644
> --- a/arch/x86/kvm/svm/svm.h
> +++ b/arch/x86/kvm/svm/svm.h
> @@ -694,6 +694,7 @@ void sev_es_vcpu_reset(struct vcpu_svm *svm);
>  void sev_vcpu_deliver_sipi_vector(struct kvm_vcpu *vcpu, u8 vector);
>  void sev_es_prepare_switch_to_guest(struct sev_es_save_area *hostsa);
>  void sev_es_unmap_ghcb(struct vcpu_svm *svm);
> +struct page *snp_safe_alloc_page(struct kvm_vcpu *vcpu);
>
>  /* vmenter.S */
>
> --
> 2.25.1
>