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

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Date: Wed, 27 Mar 2024 22:09:21 +0800
From: Binbin Wu <binbin.wu@...ux.intel.com>
To: Chenyi Qiang <chenyi.qiang@...el.com>, isaku.yamahata@...el.com
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
 isaku.yamahata@...il.com, Paolo Bonzini <pbonzini@...hat.com>,
 erdemaktas@...gle.com, Sean Christopherson <seanjc@...gle.com>,
 Sagi Shahar <sagis@...gle.com>, Kai Huang <kai.huang@...el.com>,
 chen.bo@...el.com, hang.yuan@...el.com, tina.zhang@...el.com,
 Rick Edgecombe <rick.p.edgecombe@...el.com>
Subject: Re: [PATCH v19 046/130] KVM: x86/mmu: Add address conversion
 functions for TDX shared bit of GPA



On 3/27/2024 11:08 AM, Chenyi Qiang wrote:
>
> On 2/26/2024 4:25 PM, isaku.yamahata@...el.com wrote:
>> From: Isaku Yamahata <isaku.yamahata@...el.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@...el.com>
>> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@...el.com>
>> Signed-off-by: Isaku Yamahata <isaku.yamahata@...el.com>
>> Reviewed-by: Binbin Wu <binbin.wu@...ux.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@...el.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);