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

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Message-ID: <CALMp9eRK7d1GF6Kqhji_KFz2+5jEs5JgbvyiCrqCiW_ZZiaoWQ@mail.gmail.com>
Date: Tue, 21 Oct 2025 15:11:56 -0700
From: Jim Mattson <jmattson@...gle.com>
To: Yosry Ahmed <yosry.ahmed@...ux.dev>
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 Mon, Oct 20, 2025 at 10:21 AM Yosry Ahmed <yosry.ahmed@...ux.dev> wrote:
>
> 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.

I believe it does. Would you prefer that I exit the loop before
PG_LEVEL_4K and restore the virt_get_pte() below?

> > +                     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
> >