Date:   Wed, 22 Feb 2023 22:42:53 +0200
From:   Zhi Wang <zhi.wang.linux@gmail.com>
To:     Michael Roth <michael.roth@amd.com>
Cc:     <kvm@vger.kernel.org>, <linux-coco@lists.linux.dev>,
        <linux-mm@kvack.org>, <linux-crypto@vger.kernel.org>,
        <x86@kernel.org>, <linux-kernel@vger.kernel.org>,
        <tglx@linutronix.de>, <mingo@redhat.com>, <jroedel@suse.de>,
        <thomas.lendacky@amd.com>, <hpa@zytor.com>, <ardb@kernel.org>,
        <pbonzini@redhat.com>, <seanjc@google.com>, <vkuznets@redhat.com>,
        <jmattson@google.com>, <luto@kernel.org>,
        <dave.hansen@linux.intel.com>, <slp@redhat.com>,
        <pgonda@google.com>, <peterz@infradead.org>,
        <srinivas.pandruvada@linux.intel.com>, <rientjes@google.com>,
        <dovmurik@linux.ibm.com>, <tobin@ibm.com>, <bp@alien8.de>,
        <vbabka@suse.cz>, <kirill@shutemov.name>, <ak@linux.intel.com>,
        <tony.luck@intel.com>, <marcorr@google.com>,
        <sathyanarayanan.kuppuswamy@linux.intel.com>,
        <alpergun@google.com>, <dgilbert@redhat.com>, <jarkko@kernel.org>,
        <ashish.kalra@amd.com>, <nikunj.dadhania@amd.com>,
        Brijesh Singh <brijesh.singh@amd.com>
Subject: Re: [PATCH RFC v8 31/56] KVM: SVM: Make AVIC backing, VMSA and VMCB
 memory allocation SNP safe
Message-ID: <20230222224253.00002740@gmail.com>
In-Reply-To: <20230220183847.59159-32-michael.roth@amd.com>
References: <20230220183847.59159-1-michael.roth@amd.com>
        <20230220183847.59159-32-michael.roth@amd.com>
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On Mon, 20 Feb 2023 12:38:22 -0600
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" 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@google.com>
> Signed-off-by: Marc Orr <marcorr@google.com>
> Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
> Signed-off-by: Ashish Kalra <ashish.kalra@amd.com>
> Signed-off-by: Michael Roth <michael.roth@amd.com>
> ---
>  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 */
>