linux-kernel - Re: [PATCH v3 55/60] arm64: kvm: avoid CONFIG_PGTABLE

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Message-ID: <f299a82a-a048-3f28-876d-65205f48c1b5@arm.com>
Date:   Tue, 18 Apr 2023 15:29:46 +0100
From:   Ryan Roberts <ryan.roberts@....com>
To:     Ard Biesheuvel <ardb@...nel.org>, linux-kernel@...r.kernel.org
Cc:     linux-arm-kernel@...ts.infradead.org,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>, Marc Zyngier <maz@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH v3 55/60] arm64: kvm: avoid CONFIG_PGTABLE_LEVELS for
 runtime levels

On 07/03/2023 14:05, Ard Biesheuvel wrote:
> get_user_mapping_size() uses vabits_actual and CONFIG_PGTABLE_LEVELS to
> provide the starting point for a table walk. This is fine for LVA, as
> the number of translation levels is the same regardless of whether LVA
> is enabled. However, with LPA2, this will no longer be the case, so
> let's derive the number of levels from the number of VA bits directly.
> 
> Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
> ---
>  arch/arm64/kvm/mmu.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> index d64be7b5f6692e8b..4e7c0f9a9c286c09 100644
> --- a/arch/arm64/kvm/mmu.c
> +++ b/arch/arm64/kvm/mmu.c
> @@ -663,7 +663,7 @@ static int get_user_mapping_size(struct kvm *kvm, u64 addr)
>  		.pgd		= (kvm_pteref_t)kvm->mm->pgd,
>  		.ia_bits	= vabits_actual,
>  		.start_level	= (KVM_PGTABLE_MAX_LEVELS -
> -				   CONFIG_PGTABLE_LEVELS),
> +				   ARM64_HW_PGTABLE_LEVELS(pgt.ia_bits)),
>  		.mm_ops		= &kvm_user_mm_ops,
>  	};
>  	kvm_pte_t pte = 0;	/* Keep GCC quiet... */

You have the problem here that the KVM library (which isn't LPA2 aware) is
walking a kernel page table, which may now be in LPA2 format. I think this works
out ok as long as there are no physical addresses above 48 bits in the page
table. But otherwise, I doubt it works out very well...