Re: [RFC PATCH 5/8] KVM: x86/mmu: Introduce kvm_mmu_map_page() for prepopulating guest memory

Sean Christopherson
2024-03-11T17:29:03Z

On Fri, Mar 01, 2024, isaku.yamahata@intel.com wrote:
> From: Isaku Yamahata <isaku.yamahata@intel.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@intel.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
>