linux-kernel - Re: [PATCH v2 2/4] KVM: selftests: Use a loop to walk guest page tables

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Message-ID: <6ya5ya2bzl5mvpc25fj5o62hvmubi4egs246pbj6z2oocff3tv@b3pkl6ckflpt>
Date: Thu, 30 Oct 2025 23:32:16 +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>, Marc Zyngier <maz@...nel.org>, 
	Oliver Upton <oliver.upton@...ux.dev>, Joey Gouly <joey.gouly@....com>, 
	Suzuki K Poulose <suzuki.poulose@....com>, Zenghui Yu <yuzenghui@...wei.com>, 
	Andrew Jones <ajones@...tanamicro.com>, Huacai Chen <chenhuacai@...nel.org>, 
	Bibo Mao <maobibo@...ngson.cn>, "Pratik R. Sampat" <prsampat@....com>, 
	James Houghton <jthoughton@...gle.com>, linux-kernel@...r.kernel.org, kvm@...r.kernel.org, 
	linux-kselftest@...r.kernel.org, linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev
Subject: Re: [PATCH v2 2/4] KVM: selftests: Use a loop to walk guest page
 tables

On Tue, Oct 28, 2025 at 03:30:40PM -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>

Reviewed-by: Yosry Ahmed <yosry.ahmed@...ux.dev>

> ---
>  .../testing/selftests/kvm/lib/x86/processor.c | 23 ++++++++-----------
>  1 file changed, 10 insertions(+), 13 deletions(-)
> 
> diff --git a/tools/testing/selftests/kvm/lib/x86/processor.c b/tools/testing/selftests/kvm/lib/x86/processor.c
> index 738f2a44083f..720c678187b5 100644
> --- a/tools/testing/selftests/kvm/lib/x86/processor.c
> +++ b/tools/testing/selftests/kvm/lib/x86/processor.c
> @@ -307,7 +307,8 @@ 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");
> @@ -328,19 +329,15 @@ 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 > PG_LEVEL_4K;
> +	     current_level--) {
> +		pte = virt_get_pte(vm, pte, vaddr, current_level);
> +		if (vm_is_target_pte(pte, level, current_level))
> +			return pte;
> +	}
>  
> -	return virt_get_pte(vm, pde, vaddr, PG_LEVEL_4K);
> +	return virt_get_pte(vm, pte, vaddr, PG_LEVEL_4K);
>  }
>  
>  uint64_t *vm_get_page_table_entry(struct kvm_vm *vm, uint64_t vaddr)
> -- 
> 2.51.1.851.g4ebd6896fd-goog
>