On Thu, Sep 23, 2021 at 11:09 AM Brijesh Singh <brijesh.singh@xxxxxxx> wrote: > > > On 9/22/21 1:55 PM, Dr. David Alan Gilbert wrote: > > * Brijesh Singh (brijesh.singh@xxxxxxx) wrote: > >> 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 > >> done for _all_ VMs, not just SNP-Active VMs. > > Can you explain what 'globally enabled' means? > > This means that SNP is enabled in host SYSCFG_MSR.Snp=1. Once its > enabled then RMP checks are enforced. > > > > Or more specifically, can we trip this bug on public hardware that has > > the SNP enabled in the bios, but no SNP init in the host OS? > > Enabling the SNP support on host is 3 step process: > > step1 (bios): reserve memory for the RMP table. > > step2 (host): initialize the RMP table memory, set the SYSCFG msr to > enable the SNP feature > > step3 (host): call the SNP_INIT to initialize the SNP firmware (this is > needed only if you ever plan to launch SNP guest from this host). > > The "SNP globally enabled" means the step 1 to 2. The RMP checks are > enforced as soon as step 2 is completed. It might be good to update the code to reflect this reply, such that the new `snp_safe_alloc_page()` function introduced in this patch only deviates from "normal" page allocation logic when these two conditions are met. We could introduce a global variable, `snp_globally_enabled` that defaults to false and gets set to true after we write the SYSCFG MSR. > thanks > > > > > Dave > > > >> 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 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> > >> --- > >> 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/sev.c | 35 ++++++++++++++++++++++++++++++ > >> arch/x86/kvm/svm/svm.c | 16 ++++++++++++-- > >> arch/x86/kvm/svm/svm.h | 1 + > >> 6 files changed, 56 insertions(+), 3 deletions(-) > >> > >> diff --git a/arch/x86/include/asm/kvm-x86-ops.h b/arch/x86/include/asm/kvm-x86-ops.h > >> index a12a4987154e..36a9c23a4b27 100644 > >> --- a/arch/x86/include/asm/kvm-x86-ops.h > >> +++ b/arch/x86/include/asm/kvm-x86-ops.h > >> @@ -122,6 +122,7 @@ KVM_X86_OP_NULL(enable_direct_tlbflush) > >> KVM_X86_OP_NULL(migrate_timers) > >> KVM_X86_OP(msr_filter_changed) > >> KVM_X86_OP_NULL(complete_emulated_msr) > >> +KVM_X86_OP(alloc_apic_backing_page) > >> > >> #undef KVM_X86_OP > >> #undef KVM_X86_OP_NULL > >> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h > >> index 974cbfb1eefe..5ad6255ff5d5 100644 > >> --- a/arch/x86/include/asm/kvm_host.h > >> +++ b/arch/x86/include/asm/kvm_host.h > >> @@ -1453,6 +1453,7 @@ struct kvm_x86_ops { > >> int (*complete_emulated_msr)(struct kvm_vcpu *vcpu, int err); > >> > >> void (*vcpu_deliver_sipi_vector)(struct kvm_vcpu *vcpu, u8 vector); > >> + 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 ba5a27879f1d..05b45747b20b 100644 > >> --- a/arch/x86/kvm/lapic.c > >> +++ b/arch/x86/kvm/lapic.c > >> @@ -2457,7 +2457,10 @@ 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 1644da5fc93f..8771b878193f 100644 > >> --- a/arch/x86/kvm/svm/sev.c > >> +++ b/arch/x86/kvm/svm/sev.c > >> @@ -2703,3 +2703,38 @@ 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); Continuing my other comment, above: if we introduce a `snp_globally_enabled` var, we could use that here, rather than `cpu_feature_enabled(X86_FEATURE_SEV_SNP)`. > >> + > >> + /* > >> + * 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_to_phys(pfn), PMD_SIZE)) { > >> + pfn++; > >> + __free_page(p); > >> + } else { > >> + __free_page(pfn_to_page(pfn + 1)); > >> + } > >> + > >> + return pfn_to_page(pfn); > >> +} > >> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c > >> index 25773bf72158..058eea8353c9 100644 > >> --- a/arch/x86/kvm/svm/svm.c > >> +++ b/arch/x86/kvm/svm/svm.c > >> @@ -1368,7 +1368,7 @@ static int svm_create_vcpu(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; > >> > >> @@ -1377,7 +1377,7 @@ static int svm_create_vcpu(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; > >> > >> @@ -4539,6 +4539,16 @@ 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 = { > >> .hardware_unsetup = svm_hardware_teardown, > >> .hardware_enable = svm_hardware_enable, > >> @@ -4667,6 +4677,8 @@ static struct kvm_x86_ops svm_x86_ops __initdata = { > >> .complete_emulated_msr = svm_complete_emulated_msr, > >> > >> .vcpu_deliver_sipi_vector = svm_vcpu_deliver_sipi_vector, > >> + > >> + .alloc_apic_backing_page = svm_alloc_apic_backing_page, > >> }; > >> > >> static struct kvm_x86_init_ops svm_init_ops __initdata = { > >> diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h > >> index d1f1512a4b47..e40800e9c998 100644 > >> --- a/arch/x86/kvm/svm/svm.h > >> +++ b/arch/x86/kvm/svm/svm.h > >> @@ -575,6 +575,7 @@ void sev_es_create_vcpu(struct vcpu_svm *svm); > >> void sev_vcpu_deliver_sipi_vector(struct kvm_vcpu *vcpu, u8 vector); > >> void sev_es_prepare_guest_switch(struct vcpu_svm *svm, unsigned int cpu); > >> void sev_es_unmap_ghcb(struct vcpu_svm *svm); > >> +struct page *snp_safe_alloc_page(struct kvm_vcpu *vcpu); > >> > >> /* vmenter.S */ > >> > >> -- > >> 2.17.1 > >> > >>