Re: [PATCH RFC v8 31/56] KVM: SVM: Make AVIC backing, VMSA and VMCB memory allocation SNP safe

From: Zhi Wang
Date: Wed Feb 22 2023 - 15:43:06 EST

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On Mon, 20 Feb 2023 12:38:22 -0600
Michael Roth <michael.roth@xxxxxxx> wrote:

> From: Brijesh Singh <brijesh.singh@xxxxxxx>
>
> 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" in the RMP after a successful VMRUN. This

Is this "in-use" bit part of an RMP entry? If yes, better list its name
in APM.

> is done for _all_ VMs, not just SNP-Active VMs.
_All_ VMs? Do you mean SEV VMs and SEVSNP VMs? I guess legacy VM is not
affected, right?
>
> 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 hupage, the CPU will
^hugepage
> incorrectly treat the entire 2mb region as in-use and signal a spurious
> RMP violation #PF.
>
> The recommended is to not use the hugepage for the VMCB, VMSA or
> AVIC backing page. Add a generic allocator that will ensure that the
> page returns is not hugepage (2mb or 1gb) and is safe to be used when
> SEV-SNP is enabled.
>
> Co-developed-by: Marc Orr <marcorr@xxxxxxxxxx>
> Signed-off-by: Marc Orr <marcorr@xxxxxxxxxx>
> Signed-off-by: Brijesh Singh <brijesh.singh@xxxxxxx>
> Signed-off-by: Ashish Kalra <ashish.kalra@xxxxxxx>
> Signed-off-by: Michael Roth <michael.roth@xxxxxxx>
> ---
> arch/x86/include/asm/kvm-x86-ops.h | 1 +
> arch/x86/include/asm/kvm_host.h | 2 ++
> arch/x86/kvm/lapic.c | 5 ++++-
> arch/x86/kvm/svm/sev.c | 33 ++++++++++++++++++++++++++++++
> arch/x86/kvm/svm/svm.c | 15 ++++++++++++--
> arch/x86/kvm/svm/svm.h | 1 +
> 6 files changed, 54 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/include/asm/kvm-x86-ops.h b/arch/x86/include/asm/kvm-x86-ops.h
> index 6a885f024a00..e116405cbb5f 100644
> --- a/arch/x86/include/asm/kvm-x86-ops.h
> +++ b/arch/x86/include/asm/kvm-x86-ops.h
> @@ -131,6 +131,7 @@ KVM_X86_OP(msr_filter_changed)
> 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(alloc_apic_backing_page)
> KVM_X86_OP_OPTIONAL_RET0(fault_is_private);
> KVM_X86_OP_OPTIONAL_RET0(update_mem_attr)
> KVM_X86_OP_OPTIONAL(invalidate_restricted_mem)
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 37c92412035f..a9363a6f779d 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -1729,6 +1729,8 @@ struct kvm_x86_ops {
> * Returns vCPU specific APICv inhibit reasons
> */
> unsigned long (*vcpu_get_apicv_inhibit_reasons)(struct kvm_vcpu *vcpu);
> +
> + 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 80f92cbc4029..72e46d5b4201 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -2740,7 +2740,10 @@ 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/sev.c b/arch/x86/kvm/svm/sev.c
> index c1f0d4898ce3..9e9efb42a766 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -3241,3 +3241,36 @@ void sev_vcpu_deliver_sipi_vector(struct kvm_vcpu *vcpu, u8 vector)
> break;
> }
> }
> +
> +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 VMCB, VMSA
> + * or AVIC backing page. The recommeded workaround is to not use the
> + * hugepage.
> + *
> + * Allocate one extra page, use 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;
> +}

The duplicate allocation routine in snp_alloc_vmsa_page() in sev.c can
be replaced with snp_safe_alloc_page().

> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index 213593dbd7a1..1061aaf66f0a 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -1372,7 +1372,7 @@ 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;
>
> @@ -1381,7 +1381,7 @@ 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;
>
> @@ -4696,6 +4696,16 @@ 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,
>
> @@ -4824,6 +4834,7 @@ 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 c249c360fe36..5efcf036ccad 100644
> --- a/arch/x86/kvm/svm/svm.h
> +++ b/arch/x86/kvm/svm/svm.h
> @@ -692,6 +692,7 @@ 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 */
>

Next message: Stephen Boyd: "Re: [PATCH] regulator: core: Use ktime_get_boottime() to determine how long a regulator was off"
Previous message: Sean Anderson: "[PATCH net] net: sunhme: Fix region request"
In reply to: Michael Roth: "[PATCH RFC v8 31/56] KVM: SVM: Make AVIC backing, VMSA and VMCB memory allocation SNP safe"
Next in thread: Michael Roth: "[PATCH RFC v8 33/56] KVM: SVM: Add KVM_SNP_INIT command"
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