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

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Message-ID: <CAMj1kXF2qYOgHKKK943jvYgOLcRVMwDuA9PGqDwerzhF83-QEQ@mail.gmail.com>
Date: Sat, 21 Dec 2024 00:41:39 +0100
From: Ard Biesheuvel <ardb@...nel.org>
To: Klara Modin <klarasmodin@...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>
Subject: Re: [PATCH v3 6/6] arm64/mm: Drop configurable 48-bit physical
 address space limit

On Sat, 21 Dec 2024 at 00:39, Klara Modin <klarasmodin@...il.com> wrote:
>
> Hi,
>
> On 2024-12-12 09:18, 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.
> >
>
> With this patch (32d053d6f5e92efd82349e7c481cba5a43dc1a22 in
> next-20241220), my Raspberry Pi 3 won't boot unless I set it to use
> 52-bit virtual address space (i.e. neither 39 or 48 work with a 4 KiB
> page size), nothing appears on the serial console. I didn't see anyghing
> suspicious in the kernel log for the 52-bit case but I attached it as I
> don't exactly have much else.
>
> I see that 52 bit physical address space previously depended on either
> 64 KiB pages or 52 bit virtual address space, could that be related?
>
> Please let me know if there's anything else you need.
>

Thanks for the report. This patch has already been dropped from the
arm64 tree, so it should be gone from linux-next once it gets
regenerated.