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

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Message-ID: <CAMj1kXH8W=o7yqZjL1QHud8sfA5=-aJ9HGO1opPhwihdi_28pw@mail.gmail.com>
Date: Sun, 22 Dec 2024 16:35:06 +0100
From: Ard Biesheuvel <ardb@...nel.org>
To: Nick Chan <towinchenmi@...il.com>
Cc: Ard Biesheuvel <ardb+git@...gle.com>, linux-arm-kernel@...ts.infradead.org, 
	linux-kernel@...r.kernel.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

On Sun, 22 Dec 2024 at 13:05, Nick Chan <towinchenmi@...il.com> wrote:
>
> 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.
>

Thanks for the report.

This is a known issue and has been fixed already in the arm64 tree a
couple of days ago.

Once linux-next is regenerated, things should start working again.