Re: [PATCH v6 04/41] arm64: mm: Move PCI I/O emulation region above the vmemmap region

From: Mark Rutland <mark.rutland@arm.com>
To: Ard Biesheuvel <ardb@google.com>
Cc: linux-arm-kernel@lists.infradead.org,
	Ard Biesheuvel <ardb@kernel.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>, Marc Zyngier <maz@kernel.org>,
	Ryan Roberts <ryan.roberts@arm.com>,
	Anshuman Khandual <anshuman.khandual@arm.com>,
	Kees Cook <keescook@chromium.org>
Subject: Re: [PATCH v6 04/41] arm64: mm: Move PCI I/O emulation region above the vmemmap region
Date: Mon, 11 Dec 2023 13:57:44 +0000	[thread overview]
Message-ID: <ZXcVWFMr6eUWtEec@FVFF77S0Q05N.cambridge.arm.com> (raw)
In-Reply-To: <20231129111555.3594833-47-ardb@google.com>

Hi Ard,

On Wed, Nov 29, 2023 at 12:16:00PM +0100, Ard Biesheuvel wrote:
> From: Ard Biesheuvel <ardb@kernel.org>
> 
> Move the PCI I/O region above the vmemmap region in the kernel's VA
> space. This will permit us to reclaim the lower part of the vmemmap
> region for vmalloc/vmap allocations when running a 52-bit VA capable
> build on a 48-bit VA capable system.
> 
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> ---
>  arch/arm64/include/asm/memory.h | 4 ++--
>  arch/arm64/mm/ptdump.c          | 4 ++--
>  2 files changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/memory.h b/arch/arm64/include/asm/memory.h
> index b8d726f951ae..99caeff78e1a 100644
> --- a/arch/arm64/include/asm/memory.h
> +++ b/arch/arm64/include/asm/memory.h
> @@ -49,8 +49,8 @@
>  #define MODULES_VSIZE		(SZ_2G)
>  #define VMEMMAP_START		(-(UL(1) << (VA_BITS - VMEMMAP_SHIFT)))
>  #define VMEMMAP_END		(VMEMMAP_START + VMEMMAP_SIZE)
> -#define PCI_IO_END		(VMEMMAP_START - SZ_8M)
> -#define PCI_IO_START		(PCI_IO_END - PCI_IO_SIZE)
> +#define PCI_IO_START		(VMEMMAP_END + SZ_8M)
> +#define PCI_IO_END		(PCI_IO_START + PCI_IO_SIZE)
>  #define FIXADDR_TOP		(VMEMMAP_START - SZ_32M)

This looks simple, but it does have a subtle dependency on VMEMMAP_SIZE being
sufficiently smaller than the negative offset we choose for VMEMMAP_START, and
it took mw a while to convince myself that we always ensure that today. I could
well imagine that we try to reduce that in future (given for most
configurations we're reserving around double the space we actually need), and
it would be nice if we could avoid that going wrong.

We need to make sure that PCI_IO_START and PCI_IO_END fall between VMEMMAP_END
and the last page of the VA space (due to ERR_PTR). Could we add some comment
and/or assertions to that effect?

Mark.

>  
>  #if VA_BITS > 48
> diff --git a/arch/arm64/mm/ptdump.c b/arch/arm64/mm/ptdump.c
> index e305b6593c4e..d1df56d44f8a 100644
> --- a/arch/arm64/mm/ptdump.c
> +++ b/arch/arm64/mm/ptdump.c
> @@ -47,10 +47,10 @@ static struct addr_marker address_markers[] = {
>  	{ VMALLOC_END,			"vmalloc() end" },
>  	{ FIXADDR_TOT_START,		"Fixmap start" },
>  	{ FIXADDR_TOP,			"Fixmap end" },
> -	{ PCI_IO_START,			"PCI I/O start" },
> -	{ PCI_IO_END,			"PCI I/O end" },
>  	{ VMEMMAP_START,		"vmemmap start" },
>  	{ VMEMMAP_START + VMEMMAP_SIZE,	"vmemmap end" },
> +	{ PCI_IO_START,			"PCI I/O start" },
> +	{ PCI_IO_END,			"PCI I/O end" },
>  	{ -1,				NULL },
>  };
>  
> -- 
> 2.43.0.rc1.413.gea7ed67945-goog
> 

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