Re: [PATCH v19 046/130] KVM: x86/mmu: Add address conversion functions for TDX shared bit of GPA

Binbin Wu
2024-03-27T14:09:28Z

On 3/27/2024 11:08 AM, Chenyi Qiang wrote:
>
> On 2/26/2024 4:25 PM, isaku.yamahata@intel.com wrote:
>> From: Isaku Yamahata <isaku.yamahata@intel.com>
>>
>> TDX repurposes one GPA bit (51 bit or 47 bit based on configuration) to
>> indicate the GPA is private(if cleared) or shared (if set) with VMM. If
>> GPA.shared is set, GPA is covered by the existing conventional EPT pointed
>> by EPTP. If GPA.shared bit is cleared, GPA is covered by TDX module.
>> VMM has to issue SEAMCALLs to operate.
>>
>> Add a member to remember GPA shared bit for each guest TDs, add address
>> conversion functions between private GPA and shared GPA and test if GPA
>> is private.
>>
>> Because struct kvm_arch (or struct kvm which includes struct kvm_arch. See
>> kvm_arch_alloc_vm() that passes __GPF_ZERO) is zero-cleared when allocated,
>> the new member to remember GPA shared bit is guaranteed to be zero with
>> this patch unless it's initialized explicitly.
>>
>> default or SEV-SNP TDX: S = (47 or 51) - 12
>> gfn_shared_mask 0 S bit
>> kvm_is_private_gpa() always false true if GFN has S bit set
> TDX: true if GFN has S bit clear?
>
>> kvm_gfn_to_shared() nop set S bit
>> kvm_gfn_to_private() nop clear S bit
>>
>> fault.is_private means that host page should be gotten from guest_memfd
>> is_private_gpa() means that KVM MMU should invoke private MMU hooks.
>>
>> Co-developed-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
>> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
>> Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>
>> Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com>
>> ---
>> v19:
>> - Add comment on default vm case.
>> - Added behavior table in the commit message
>> - drop CONFIG_KVM_MMU_PRIVATE
>>
>> v18:
>> - Added Reviewed-by Binbin
>>
>> Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>
>> ---
>> arch/x86/include/asm/kvm_host.h | 2 ++
>> arch/x86/kvm/mmu.h | 33 +++++++++++++++++++++++++++++++++
>> arch/x86/kvm/vmx/tdx.c | 5 +++++
>> 3 files changed, 40 insertions(+)
>>
>> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
>> index 5da3c211955d..de6dd42d226f 100644
>> --- a/arch/x86/include/asm/kvm_host.h
>> +++ b/arch/x86/include/asm/kvm_host.h
>> @@ -1505,6 +1505,8 @@ struct kvm_arch {
>> */
>> #define SPLIT_DESC_CACHE_MIN_NR_OBJECTS (SPTE_ENT_PER_PAGE + 1)
>> struct kvm_mmu_memory_cache split_desc_cache;
>> +
>> + gfn_t gfn_shared_mask;
>> };
>>
>> struct kvm_vm_stat {
>> diff --git a/arch/x86/kvm/mmu.h b/arch/x86/kvm/mmu.h
>> index d96c93a25b3b..395b55684cb9 100644
>> --- a/arch/x86/kvm/mmu.h
>> +++ b/arch/x86/kvm/mmu.h
>> @@ -322,4 +322,37 @@ static inline gpa_t kvm_translate_gpa(struct kvm_vcpu *vcpu,
>> return gpa;
>> return translate_nested_gpa(vcpu, gpa, access, exception);
>> }
>> +
>> +/*
>> + * default or SEV-SNP TDX: where S = (47 or 51) - 12
>> + * gfn_shared_mask 0 S bit
>> + * is_private_gpa() always false if GPA has S bit set

Also here,
TDX: true if GFN has S bit cleared

>> + * gfn_to_shared() nop set S bit
>> + * gfn_to_private() nop clear S bit
>> + *
>> + * fault.is_private means that host page should be gotten from guest_memfd
>> + * is_private_gpa() means that KVM MMU should invoke private MMU hooks.
>> + */
>> +static inline gfn_t kvm_gfn_shared_mask(const struct kvm *kvm)
>> +{
>> + return kvm->arch.gfn_shared_mask;
>> +}
>> +
>> +static inline gfn_t kvm_gfn_to_shared(const struct kvm *kvm, gfn_t gfn)
>> +{
>> + return gfn | kvm_gfn_shared_mask(kvm);
>> +}
>> +
>> +static inline gfn_t kvm_gfn_to_private(const struct kvm *kvm, gfn_t gfn)
>> +{
>> + return gfn & ~kvm_gfn_shared_mask(kvm);
>> +}
>> +
>> +static inline bool kvm_is_private_gpa(const struct kvm *kvm, gpa_t gpa)
>> +{
>> + gfn_t mask = kvm_gfn_shared_mask(kvm);
>> +
>> + return mask && !(gpa_to_gfn(gpa) & mask);
>> +}
>> +
>> #endif
>> diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
>> index aa1da51b8af7..54e0d4efa2bd 100644
>> --- a/arch/x86/kvm/vmx/tdx.c
>> +++ b/arch/x86/kvm/vmx/tdx.c
>> @@ -906,6 +906,11 @@ static int tdx_td_init(struct kvm *kvm, struct kvm_tdx_cmd *cmd)
>> kvm_tdx->attributes = td_params->attributes;
>> kvm_tdx->xfam = td_params->xfam;
>>
>> + if (td_params->exec_controls & TDX_EXEC_CONTROL_MAX_GPAW)
>> + kvm->arch.gfn_shared_mask = gpa_to_gfn(BIT_ULL(51));
>> + else
>> + kvm->arch.gfn_shared_mask = gpa_to_gfn(BIT_ULL(47));
>> +
>> out:
>> /* kfree() accepts NULL. */
>> kfree(init_vm);