linux-kernel - Re: [PATCH v3 6/6] arm64/mm: Drop configurable 48-bit physical address space limit

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Message-ID: <bf93c563-f331-43c0-a145-1abf143bd0eb@gmail.com>
Date: Sun, 22 Dec 2024 20:05:28 +0800
From: Nick Chan <towinchenmi@...il.com>
To: Ard Biesheuvel <ardb+git@...gle.com>, linux-arm-kernel@...ts.infradead.org
Cc: linux-kernel@...r.kernel.org, Ard Biesheuvel <ardb@...nel.org>,
 Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>,
 Marc Zyngier <maz@...nel.org>, Mark Rutland <mark.rutland@....com>,
 Ryan Roberts <ryan.roberts@....com>,
 Anshuman Khandual <anshuman.khandual@....com>,
 Kees Cook <keescook@...omium.org>, Quentin Perret <qperret@...gle.com>,
 asahi@...ts.linux.dev
Subject: Re: [PATCH v3 6/6] arm64/mm: Drop configurable 48-bit physical
 address space limit

Hi Ard,

On 12/12/2024 16:18, Ard Biesheuvel wrote:
> From: Ard Biesheuvel <ardb@...nel.org>
> 
> Currently, the maximum supported physical address space can be
> configured as either 48 bits or 52 bits. The only remaining difference
> between these in practice is that the former omits the masking and
> shifting required to construct TTBR and PTE values, which carry bits #48
> and higher disjoint from the rest of the physical address.
> 
> The overhead of performing these additional calculations is negligible,
> and so there is little reason to retain support for two different
> configurations, and we can simply support whatever the hardware
> supports.

I am seeing a boot failure on Apple iPad 7 which uses 
CONFIG_ARM64_16K=y after this change in linux-next as commit
32d053d6f5e9, with nothing appearing on serial console with
earlycon enabled unless I set CONFIG_ARM64_VA_BITS_52=y. Reverting
this patch makes the kernel work again.

Nick Chan