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

[Chenyi Qiang]

On 2/26/2024 4:25 PM, isaku.yamahata@xxxxxxxxx wrote:
> From: Isaku Yamahata <isaku.yamahata@xxxxxxxxx>
> 
> 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@xxxxxxxxx>
> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
> Signed-off-by: Isaku Yamahata <isaku.yamahata@xxxxxxxxx>
> Reviewed-by: Binbin Wu <binbin.wu@xxxxxxxxxxxxxxx>
> ---
> 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@xxxxxxxxx>
> ---
>  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
> + * 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);