linux-kernel - Re: [PATCH 1/4] KVM: selftests: Use a loop to create guest page tables

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Message-ID: <bbppzosz2alycri5o75fibahsqeb2y4bq5n6b3b23amrgtlrro@ppp5vwu2uoat>
Date: Mon, 20 Oct 2025 17:10:50 +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 1/4] KVM: selftests: Use a loop to create guest page
 tables

On Wed, Sep 17, 2025 at 02:48:37PM -0700, Jim Mattson wrote:
> Walk the guest page tables via a loop when creating new mappings,
> 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 | 22 +++++++------------
>  1 file changed, 8 insertions(+), 14 deletions(-)
> 
> diff --git a/tools/testing/selftests/kvm/lib/x86/processor.c b/tools/testing/selftests/kvm/lib/x86/processor.c
> index d4c19ac885a9..0238e674709d 100644
> --- a/tools/testing/selftests/kvm/lib/x86/processor.c
> +++ b/tools/testing/selftests/kvm/lib/x86/processor.c
> @@ -184,8 +184,8 @@ static uint64_t *virt_create_upper_pte(struct kvm_vm *vm,
>  void __virt_pg_map(struct kvm_vm *vm, uint64_t vaddr, uint64_t paddr, int level)
>  {
>  	const uint64_t pg_size = PG_LEVEL_SIZE(level);
> -	uint64_t *pml4e, *pdpe, *pde;
> -	uint64_t *pte;
> +	uint64_t *pte = &vm->pgd;
> +	int current_level;
>  
>  	TEST_ASSERT(vm->mode == VM_MODE_PXXV48_4K,
>  		    "Unknown or unsupported guest mode, mode: 0x%x", vm->mode);
> @@ -209,20 +209,14 @@ void __virt_pg_map(struct kvm_vm *vm, uint64_t vaddr, uint64_t paddr, int level)
>  	 * Allocate upper level page tables, if not already present.  Return
>  	 * early if a hugepage was created.
>  	 */
> -	pml4e = virt_create_upper_pte(vm, &vm->pgd, vaddr, paddr, PG_LEVEL_512G, level);
> -	if (*pml4e & PTE_LARGE_MASK)
> -		return;
> -
> -	pdpe = virt_create_upper_pte(vm, pml4e, vaddr, paddr, PG_LEVEL_1G, level);
> -	if (*pdpe & PTE_LARGE_MASK)
> -		return;
> -
> -	pde = virt_create_upper_pte(vm, pdpe, vaddr, paddr, PG_LEVEL_2M, level);
> -	if (*pde & PTE_LARGE_MASK)
> -		return;
> +	for (current_level = vm->pgtable_levels; current_level > 0; current_level--) {

I think the condition here should be: "current_level > PG_LEVEL_4K" or
"current_level >= PG_LEVEL_2M". PG_LEVEL_4K is 1, so right now we will
call virt_create_upper_pte() for PG_LEVEL_4K and skip the logic after
the logic after the loop.

I think it still accidentally works for most cases, but we shouldn't
rely on that.

> +		pte = virt_create_upper_pte(vm, pte, vaddr, paddr, current_level, level);
> +		if (*pte & PTE_LARGE_MASK)
> +			return;
> +	}
>  
>  	/* Fill in page table entry. */
> -	pte = virt_get_pte(vm, pde, vaddr, PG_LEVEL_4K);
> +	pte = virt_get_pte(vm, pte, vaddr, PG_LEVEL_4K);
>  	TEST_ASSERT(!(*pte & PTE_PRESENT_MASK),
>  		    "PTE already present for 4k page at vaddr: 0x%lx", vaddr);
>  	*pte = PTE_PRESENT_MASK | PTE_WRITABLE_MASK | (paddr & PHYSICAL_PAGE_MASK);
> -- 
> 2.51.0.470.ga7dc726c21-goog
>