linux-kernel - Re: [RFC PATCH 5/8] KVM: x86/mmu: Introduce kvm_mmu_map

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Date: Mon, 11 Mar 2024 10:29:01 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: isaku.yamahata@...el.com
Cc: kvm@...r.kernel.org, isaku.yamahata@...il.com, 
	linux-kernel@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>, 
	Michael Roth <michael.roth@....com>, David Matlack <dmatlack@...gle.com>, 
	Federico Parola <federico.parola@...ito.it>
Subject: Re: [RFC PATCH 5/8] KVM: x86/mmu: Introduce kvm_mmu_map_page() for
 prepopulating guest memory

On Fri, Mar 01, 2024, isaku.yamahata@...el.com wrote:
> From: Isaku Yamahata <isaku.yamahata@...el.com>
> 
> Introduce a helper function to call kvm fault handler.  This allows
> a new ioctl to invoke kvm fault handler to populate without seeing
> RET_PF_* enums or other KVM MMU internal definitions.
> 
> Signed-off-by: Isaku Yamahata <isaku.yamahata@...el.com>
> ---
>  arch/x86/kvm/mmu.h     |  3 +++
>  arch/x86/kvm/mmu/mmu.c | 30 ++++++++++++++++++++++++++++++
>  2 files changed, 33 insertions(+)
> 
> diff --git a/arch/x86/kvm/mmu.h b/arch/x86/kvm/mmu.h
> index 60f21bb4c27b..48870c5e08ec 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_mmu_map_page(struct kvm_vcpu *vcpu, gpa_t gpa, u32 error_code,
> +		     u8 max_level, u8 *goal_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 e4cc7f764980..7d5e80d17977 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -4659,6 +4659,36 @@ int kvm_tdp_page_fault(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
>  	return direct_page_fault(vcpu, fault);
>  }
>  
> +int kvm_mmu_map_page(struct kvm_vcpu *vcpu, gpa_t gpa, u32 error_code,
> +		     u8 max_level, u8 *goal_level)
> +{
> +	struct kvm_page_fault fault = KVM_PAGE_FAULT_INIT(vcpu, gpa, error_code,
> +							  false, max_level);
> +	int r;
> +
> +	r = __kvm_mmu_do_page_fault(vcpu, &fault);
> +
> +	if (is_error_noslot_pfn(fault.pfn) || vcpu->kvm->vm_bugged)
> +		return -EFAULT;

This clobbers a non-zero 'r'.  And KVM return -EIO if the VM is bugged/dead, not
-EFAULT.  I also don't see why KVM needs to explicitly check is_error_noslot_pfn(),
that should be funneled to RET_PF_EMULATE.

> +
> +	switch (r) {
> +	case RET_PF_RETRY:
> +		return -EAGAIN;
> +
> +	case RET_PF_FIXED:
> +	case RET_PF_SPURIOUS:
> +		*goal_level = fault.goal_level;
> +		return 0;
> +
> +	case RET_PF_CONTINUE:
> +	case RET_PF_EMULATE:

-EINVAL woud be more appropriate for RET_PF_EMULATE.

> +	case RET_PF_INVALID:

CONTINUE and INVALID should be WARN conditions.

> +	default:
> +		return -EIO;
> +	}
> +}
> +EXPORT_SYMBOL_GPL(kvm_mmu_map_page);

Unnecessary export.

> +
>  static void nonpaging_init_context(struct kvm_mmu *context)
>  {
>  	context->page_fault = nonpaging_page_fault;
> -- 
> 2.25.1
>