Re: [PATCH v1 1/3] kvm: svm: Add support for additional SVM NPF error codes
From: Tom Lendacky
Date: Tue Nov 22 2016 - 17:15:37 EST
On 11/21/2016 9:12 AM, Paolo Bonzini wrote:
>
>
> On 14/11/2016 23:15, Brijesh Singh wrote:
>> From: Tom Lendacky <thomas.lendacky@xxxxxxx>
>>
>> AMD hardware adds two additional bits to aid in nested page fault handling.
>>
>> Bit 32 - NPF occurred while translating the guest's final physical address
>> Bit 33 - NPF occurred while translating the guest page tables
>
> I have two questions out of curiosity, and to better understand the
> differences between Intel and AMD:
I talked with some folks about these questions and here's what we
determined:
>
> 1) are the two bits mutually exclusive, and is one bit always set?
The two bits are mutually exclusive - either the processor encounters
the fault while translating the final gPA or while translating a guest
page table, there's no way for it to be both.
>
> 2) what bit is set if the processor is reading the PDPTEs of a 32-bit
> PAE guest?
I believe that bit 33 will be set. The PDPE's are considered guest
tables and are read during a guest table walk (see APM vol2 section
15.25.10). Note that this is slightly different than the bare-metal
behavior of legacy PAE mode as APM describes. I'll try to test this
and verify it.
Thanks,
Tom
>
> Thanks,
>
> Paolo
>
>> The guest page tables fault indicator can be used as an aid for nested
>> virtualization. Using V0 for the host, V1 for the first level guest and
>> V2 for the second level guest, when both V1 and V2 are using nested paging
>> there are currently a number of unnecessary instruction emulations. When
>> V2 is launched shadow paging is used in V1 for the nested tables of V2. As
>> a result, KVM marks these pages as RO in the host nested page tables. When
>> V2 exits and we resume V1, these pages are still marked RO.
>>
>> Every nested walk for a guest page table is treated as a user-level write
>> access and this causes a lot of NPFs because the V1 page tables are marked
>> RO in the V0 nested tables. While executing V1, when these NPFs occur KVM
>> sees a write to a read-only page, emulates the V1 instruction and unprotects
>> the page (marking it RW). This patch looks for cases where we get a NPF due
>> to a guest page table walk where the page was marked RO. It immediately
>> unprotects the page and resumes the guest, leading to far fewer instruction
>> emulations when nested virtualization is used.
>>
>> Signed-off-by: Tom Lendacky <thomas.lendacky@xxxxxxx>
>> Reviewed-by: Borislav Petkov <bp@xxxxxxx>
>> Signed-off-by: Brijesh Singh <brijesh.singh@xxxxxxx>
>> ---
>> arch/x86/include/asm/kvm_host.h | 11 ++++++++++-
>> arch/x86/kvm/mmu.c | 20 ++++++++++++++++++--
>> arch/x86/kvm/svm.c | 2 +-
>> 3 files changed, 29 insertions(+), 4 deletions(-)
>>
>> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
>> index bdde807..da07e17 100644
>> --- a/arch/x86/include/asm/kvm_host.h
>> +++ b/arch/x86/include/asm/kvm_host.h
>> @@ -191,6 +191,8 @@ enum {
>> #define PFERR_RSVD_BIT 3
>> #define PFERR_FETCH_BIT 4
>> #define PFERR_PK_BIT 5
>> +#define PFERR_GUEST_FINAL_BIT 32
>> +#define PFERR_GUEST_PAGE_BIT 33
>>
>> #define PFERR_PRESENT_MASK (1U << PFERR_PRESENT_BIT)
>> #define PFERR_WRITE_MASK (1U << PFERR_WRITE_BIT)
>> @@ -198,6 +200,13 @@ enum {
>> #define PFERR_RSVD_MASK (1U << PFERR_RSVD_BIT)
>> #define PFERR_FETCH_MASK (1U << PFERR_FETCH_BIT)
>> #define PFERR_PK_MASK (1U << PFERR_PK_BIT)
>> +#define PFERR_GUEST_FINAL_MASK (1ULL << PFERR_GUEST_FINAL_BIT)
>> +#define PFERR_GUEST_PAGE_MASK (1ULL << PFERR_GUEST_PAGE_BIT)
>> +
>> +#define PFERR_NESTED_GUEST_PAGE (PFERR_GUEST_PAGE_MASK | \
>> + PFERR_USER_MASK | \
>> + PFERR_WRITE_MASK | \
>> + PFERR_PRESENT_MASK)
>>
>> /* apic attention bits */
>> #define KVM_APIC_CHECK_VAPIC 0
>> @@ -1203,7 +1212,7 @@ void kvm_vcpu_deactivate_apicv(struct kvm_vcpu *vcpu);
>>
>> int kvm_emulate_hypercall(struct kvm_vcpu *vcpu);
>>
>> -int kvm_mmu_page_fault(struct kvm_vcpu *vcpu, gva_t gva, u32 error_code,
>> +int kvm_mmu_page_fault(struct kvm_vcpu *vcpu, gva_t gva, u64 error_code,
>> void *insn, int insn_len);
>> void kvm_mmu_invlpg(struct kvm_vcpu *vcpu, gva_t gva);
>> void kvm_mmu_new_cr3(struct kvm_vcpu *vcpu);
>> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
>> index d9c7e98..f633d29 100644
>> --- a/arch/x86/kvm/mmu.c
>> +++ b/arch/x86/kvm/mmu.c
>> @@ -4508,7 +4508,7 @@ static void make_mmu_pages_available(struct kvm_vcpu *vcpu)
>> kvm_mmu_commit_zap_page(vcpu->kvm, &invalid_list);
>> }
>>
>> -int kvm_mmu_page_fault(struct kvm_vcpu *vcpu, gva_t cr2, u32 error_code,
>> +int kvm_mmu_page_fault(struct kvm_vcpu *vcpu, gva_t cr2, u64 error_code,
>> void *insn, int insn_len)
>> {
>> int r, emulation_type = EMULTYPE_RETRY;
>> @@ -4527,12 +4527,28 @@ int kvm_mmu_page_fault(struct kvm_vcpu *vcpu, gva_t cr2, u32 error_code,
>> return r;
>> }
>>
>> - r = vcpu->arch.mmu.page_fault(vcpu, cr2, error_code, false);
>> + r = vcpu->arch.mmu.page_fault(vcpu, cr2, lower_32_bits(error_code),
>> + false);
>> if (r < 0)
>> return r;
>> if (!r)
>> return 1;
>>
>> + /*
>> + * Before emulating the instruction, check if the error code
>> + * was due to a RO violation while translating the guest page.
>> + * This can occur when using nested virtualization with nested
>> + * paging in both guests. If true, we simply unprotect the page
>> + * and resume the guest.
>> + *
>> + * Note: AMD only (since it supports the PFERR_GUEST_PAGE_MASK used
>> + * in PFERR_NEXT_GUEST_PAGE)
>> + */
>> + if (error_code == PFERR_NESTED_GUEST_PAGE) {
>> + kvm_mmu_unprotect_page(vcpu->kvm, gpa_to_gfn(cr2));
>> + return 1;
>> + }
>> +
>> if (mmio_info_in_cache(vcpu, cr2, direct))
>> emulation_type = 0;
>> emulate:
>> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
>> index 8ca1eca..4e462bb 100644
>> --- a/arch/x86/kvm/svm.c
>> +++ b/arch/x86/kvm/svm.c
>> @@ -2074,7 +2074,7 @@ static void svm_set_dr7(struct kvm_vcpu *vcpu, unsigned long value)
>> static int pf_interception(struct vcpu_svm *svm)
>> {
>> u64 fault_address = svm->vmcb->control.exit_info_2;
>> - u32 error_code;
>> + u64 error_code;
>> int r = 1;
>>
>> switch (svm->apf_reason) {
>>