Re: [PATCH 5/7] KVM: x86/mmu: Introduce kvm_tdp_map_page() to populate guest memory

[Sean Christopherson <seanjc@google.com>]

On Wed, Apr 17, 2024, Paolo Bonzini wrote:
> From: Isaku Yamahata <isaku.yamahata@intel.com>
> 
> Introduce a helper function to call the KVM fault handler.  It allows a new
> ioctl to invoke the KVM fault handler to populate without seeing RET_PF_*
> enums or other KVM MMU internal definitions because RET_PF_* are internal
> to x86 KVM MMU.  The implementation is restricted to two-dimensional paging
> for simplicity.  The shadow paging uses GVA for faulting instead of L1 GPA.
> It makes the API difficult to use.
> 
> Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>
> Message-ID: <9b866a0ae7147f96571c439e75429a03dcb659b6.1712785629.git.isaku.yamahata@intel.com>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>  arch/x86/kvm/mmu.h     |  3 +++
>  arch/x86/kvm/mmu/mmu.c | 32 ++++++++++++++++++++++++++++++++
>  2 files changed, 35 insertions(+)
> 
> diff --git a/arch/x86/kvm/mmu.h b/arch/x86/kvm/mmu.h
> index e8b620a85627..51ff4f67e115 100644
> --- a/arch/x86/kvm/mmu.h
> +++ b/arch/x86/kvm/mmu.h
> @@ -183,6 +183,9 @@ static inline void kvm_mmu_refresh_passthrough_bits(struct kvm_vcpu *vcpu,
>  	__kvm_mmu_refresh_passthrough_bits(vcpu, mmu);
>  }
>  
> +int kvm_tdp_map_page(struct kvm_vcpu *vcpu, gpa_t gpa, u64 error_code,
> +		     u8 *level);
> +
>  /*
>   * Check if a given access (described through the I/D, W/R and U/S bits of a
>   * page fault error code pfec) causes a permission fault with the given PTE
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 7fbcfc97edcc..fb2149d16f8d 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -4646,6 +4646,38 @@ int kvm_tdp_page_fault(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
>  	return direct_page_fault(vcpu, fault);
>  }
>  
> +int kvm_tdp_map_page(struct kvm_vcpu *vcpu, gpa_t gpa, u64 error_code,
> +		     u8 *level)

If the return is an overloaded "long", then there's no need for @level, i.e. do
the level=>size conversion in this helper.

> +{
> +	int r;
> +
> +	/* Restrict to TDP page fault. */

Do we want to restrict this to the TDP MMU?  Not for any particular reason, mostly
just to keep moving towards officially deprecating/removing TDP support from the
shadow MMU.

> +	if (vcpu->arch.mmu->page_fault != kvm_tdp_page_fault)
> +		return -EOPNOTSUPP;
> +
> +	r = __kvm_mmu_do_page_fault(vcpu, gpa, error_code, true, NULL, level);
> +	if (r < 0)
> +		return r;
> +
> +	switch (r) {
> +	case RET_PF_RETRY:
> +		return -EAGAIN;
> +
> +	case RET_PF_FIXED:
> +	case RET_PF_SPURIOUS:
> +		return 0;

Going with the "long" idea, this becomes:

		end = (gpa & KVM_HPAGE_MASK(level)) + KVM_HPAGE_SIZE(level);
		return min(size, end - gpa);

though I would vote for a:

		break;

so that the happy path is nicely isolated at the end of the function.

> +
> +	case RET_PF_EMULATE:
> +		return -EINVAL;
> +
> +	case RET_PF_CONTINUE:
> +	case RET_PF_INVALID:
> +	default:
> +		WARN_ON_ONCE(r);
> +		return -EIO;
> +	}
> +}
> +
>  static void nonpaging_init_context(struct kvm_mmu *context)
>  {
>  	context->page_fault = nonpaging_page_fault;
> -- 
> 2.43.0
> 
>