linux-kernel - Re: [PATCH 2/4] KVM: selftests: Use a loop to walk guest page tables

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Message-ID: <o7m5gh76crbgzlfvq4lbp6ymuzbgze25qphlhsezl2ox5rfjuv@3xh7gqh5dmlt>
Date: Mon, 20 Oct 2025 17:21:07 +0000
From: Yosry Ahmed <yosry.ahmed@...ux.dev>
To: Jim Mattson <jmattson@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, Shuah Khan <shuah@...nel.org>, 
	Sean Christopherson <seanjc@...gle.com>, Bibo Mao <maobibo@...ngson.cn>, 
	Huacai Chen <chenhuacai@...nel.org>, Andrew Jones <ajones@...tanamicro.com>, 
	Claudio Imbrenda <imbrenda@...ux.ibm.com>, "Pratik R. Sampat" <prsampat@....com>, 
	Kai Huang <kai.huang@...el.com>, Eric Auger <eric.auger@...hat.com>, linux-kernel@...r.kernel.org, 
	kvm@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH 2/4] KVM: selftests: Use a loop to walk guest page tables

On Wed, Sep 17, 2025 at 02:48:38PM -0700, Jim Mattson wrote:
> Walk the guest page tables via a loop when searching for a PTE,
> instead of using unique variables for each level of the page tables.
> 
> This simplifies the code and makes it easier to support 5-level paging
> in the future.
> 
> Signed-off-by: Jim Mattson <jmattson@...gle.com>
> ---
>  .../testing/selftests/kvm/lib/x86/processor.c | 21 +++++++------------
>  1 file changed, 8 insertions(+), 13 deletions(-)
> 
> diff --git a/tools/testing/selftests/kvm/lib/x86/processor.c b/tools/testing/selftests/kvm/lib/x86/processor.c
> index 0238e674709d..433365c8196d 100644
> --- a/tools/testing/selftests/kvm/lib/x86/processor.c
> +++ b/tools/testing/selftests/kvm/lib/x86/processor.c
> @@ -270,7 +270,8 @@ static bool vm_is_target_pte(uint64_t *pte, int *level, int current_level)
>  uint64_t *__vm_get_page_table_entry(struct kvm_vm *vm, uint64_t vaddr,
>  				    int *level)
>  {
> -	uint64_t *pml4e, *pdpe, *pde;
> +	uint64_t *pte = &vm->pgd;
> +	int current_level;
>  
>  	TEST_ASSERT(!vm->arch.is_pt_protected,
>  		    "Walking page tables of protected guests is impossible");
> @@ -291,19 +292,13 @@ uint64_t *__vm_get_page_table_entry(struct kvm_vm *vm, uint64_t vaddr,
>  	TEST_ASSERT(vaddr == (((int64_t)vaddr << 16) >> 16),
>  		"Canonical check failed.  The virtual address is invalid.");
>  
> -	pml4e = virt_get_pte(vm, &vm->pgd, vaddr, PG_LEVEL_512G);
> -	if (vm_is_target_pte(pml4e, level, PG_LEVEL_512G))
> -		return pml4e;
> -
> -	pdpe = virt_get_pte(vm, pml4e, vaddr, PG_LEVEL_1G);
> -	if (vm_is_target_pte(pdpe, level, PG_LEVEL_1G))
> -		return pdpe;
> -
> -	pde = virt_get_pte(vm, pdpe, vaddr, PG_LEVEL_2M);
> -	if (vm_is_target_pte(pde, level, PG_LEVEL_2M))
> -		return pde;
> +	for (current_level = vm->pgtable_levels; current_level > 0; current_level--) {

This should be current_level >= PG_LEVEL_4K. It's the same, but easier
to read.

> +		pte = virt_get_pte(vm, pte, vaddr, current_level);
> +		if (vm_is_target_pte(pte, level, current_level))

Seems like vm_is_target_pte() is written with the assumption that it
operates on an upper-level PTE, but I think it works on 4K PTEs as well.

> +			return pte;
> +	}
>  
> -	return virt_get_pte(vm, pde, vaddr, PG_LEVEL_4K);
> +	return pte;
>  }
>  
>  uint64_t *vm_get_page_table_entry(struct kvm_vm *vm, uint64_t vaddr)
> -- 
> 2.51.0.470.ga7dc726c21-goog
>