Re: [PATCH 1/6] arm64: don't override idmap t0sz

From: Ard Biesheuvel <ardb@kernel.org>
To: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: Linux ARM <linux-arm-kernel@lists.infradead.org>,
	 Mark Rutland <mark.rutland@arm.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>
Subject: Re: [PATCH 1/6] arm64: don't override idmap t0sz
Date: Wed, 18 May 2022 08:41:19 +0200	[thread overview]
Message-ID: <CAMj1kXF2ByuUMXWgtcdsJ=j9qo+dCKy=8KqCN95iJ167YLt37Q@mail.gmail.com> (raw)
In-Reply-To: <20220518031725.3128044-2-anshuman.khandual@arm.com>

On Wed, 18 May 2022 at 05:17, Anshuman Khandual
<anshuman.khandual@arm.com> wrote:
>
> From: Mark Rutland <mark.rutland@arm.com>
>
> When the kernel is built with CONFIG_ARM64_VA_BITS_52, __cpu_setup will
> override `idmap_t0sz`, and program TCR_EL1.T0SZ based on
> `vabits_actual`. This is inconsistent with cpu_set_idmap_tcr_t0sz(),
> which will use `idmap_t0sz`, but happens to work as
> CONFIG_ARM64_VA_BITS_52 requires 64K pages where 48-bit VAs and 52-bit
> VAs required the same number of page table levels and TTBR0 addresses
> grow upwards from the base of the PGD table (for which the entire page
> is zeroed).
>
> When switching away from the idmap, cpu_set_default_tcr_t0sz() will use
> `vabits_actual`, and so the T0SZ value used for the idmap does not have
> to match the T0SZ used during regular kernel/userspace execution.
>
> This patch ensures we *always* use `idmap_t0sz` as the TCR_EL1.T0SZ
> value used while the idmap is active.
>
> Cc: Ard Biesheuvel <ardb@kernel.org>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Will Deacon <will@kernel.org>
> Signed-off-by: Mark Rutland <mark.rutland@arm.com>
> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>

Acked-by: Ard Biesheuvel <ardb@kernel.org>

Note that this conflicts [trivially] with my series here:
https://lore.kernel.org/linux-arm-kernel/20220411094824.4176877-1-ardb@kernel.org/


> ---
>  arch/arm64/mm/proc.S | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
>
> diff --git a/arch/arm64/mm/proc.S b/arch/arm64/mm/proc.S
> index 50bbed947bec..c1f76bf3276c 100644
> --- a/arch/arm64/mm/proc.S
> +++ b/arch/arm64/mm/proc.S
> @@ -468,9 +468,8 @@ SYM_FUNC_START(__cpu_setup)
>         sub             x9, xzr, x9
>         add             x9, x9, #64
>         tcr_set_t1sz    tcr, x9
> -#else
> -       ldr_l           x9, idmap_t0sz
>  #endif
> +       ldr_l           x9, idmap_t0sz
>         tcr_set_t0sz    tcr, x9
>
>         /*
> --
> 2.20.1
>

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