linux-kernel - Re: [PATCH v4 06/15] arm64: Make the PHYS_MASK

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Message-ID: <20240709114337.GB13242@willie-the-truck>
Date: Tue, 9 Jul 2024 12:43:37 +0100
From: Will Deacon <will@...nel.org>
To: Steven Price <steven.price@....com>
Cc: kvm@...r.kernel.org, kvmarm@...ts.linux.dev,
	Catalin Marinas <catalin.marinas@....com>,
	Marc Zyngier <maz@...nel.org>, James Morse <james.morse@....com>,
	Oliver Upton <oliver.upton@...ux.dev>,
	Suzuki K Poulose <suzuki.poulose@....com>,
	Zenghui Yu <yuzenghui@...wei.com>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	Joey Gouly <joey.gouly@....com>,
	Alexandru Elisei <alexandru.elisei@....com>,
	Christoffer Dall <christoffer.dall@....com>,
	Fuad Tabba <tabba@...gle.com>, linux-coco@...ts.linux.dev,
	Ganapatrao Kulkarni <gankulkarni@...amperecomputing.com>
Subject: Re: [PATCH v4 06/15] arm64: Make the PHYS_MASK_SHIFT dynamic

On Mon, Jul 01, 2024 at 10:54:56AM +0100, Steven Price wrote:
> Make the PHYS_MASK_SHIFT dynamic for Realms. This is only is required
> for masking the PFN from a pte entry. For a realm phys_mask_shift is
> reduced if the RMM reports a smaller configured size for the guest.
> 
> The realm configuration splits the address space into two with the top
> half being memory shared with the host, and the bottom half being
> protected memory. We treat the bit which controls this split as an
> attribute bit and hence exclude it (and any higher bits) from the mask.
> 
> Co-developed-by: Suzuki K Poulose <suzuki.poulose@....com>
> Signed-off-by: Suzuki K Poulose <suzuki.poulose@....com>
> Signed-off-by: Steven Price <steven.price@....com>
> 
> ---
> v3: Drop the MAX_PHYS_MASK{,_SHIFT} definitions as they are no longer
> needed.
> ---
>  arch/arm64/include/asm/pgtable-hwdef.h | 6 ------
>  arch/arm64/include/asm/pgtable.h       | 5 +++++
>  arch/arm64/kernel/rsi.c                | 5 +++++
>  3 files changed, 10 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/pgtable-hwdef.h b/arch/arm64/include/asm/pgtable-hwdef.h
> index 9943ff0af4c9..2e3af0693bd8 100644
> --- a/arch/arm64/include/asm/pgtable-hwdef.h
> +++ b/arch/arm64/include/asm/pgtable-hwdef.h
> @@ -203,12 +203,6 @@
>   */
>  #define PTE_S2_MEMATTR(t)	(_AT(pteval_t, (t)) << 2)
>  
> -/*
> - * Highest possible physical address supported.
> - */
> -#define PHYS_MASK_SHIFT		(CONFIG_ARM64_PA_BITS)
> -#define PHYS_MASK		((UL(1) << PHYS_MASK_SHIFT) - 1)
> -
>  #define TTBR_CNP_BIT		(UL(1) << 0)
>  
>  /*
> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> index f8efbc128446..11d614d83317 100644
> --- a/arch/arm64/include/asm/pgtable.h
> +++ b/arch/arm64/include/asm/pgtable.h
> @@ -39,6 +39,11 @@
>  #include <linux/sched.h>
>  #include <linux/page_table_check.h>
>  
> +extern unsigned int phys_mask_shift;
> +
> +#define PHYS_MASK_SHIFT		(phys_mask_shift)
> +#define PHYS_MASK		((1UL << PHYS_MASK_SHIFT) - 1)

I tried to figure out where this is actually used so I could understand
your comment in the commit message:

 > This is only is required for masking the PFN from a pte entry

The closest thing I could find is in arch/arm64/mm/mmap.c, where the
mask is used as part of valid_mmap_phys_addr_range() which exists purely
to filter accesses to /dev/mem. That's pretty niche, so why not just
inline the RSI-specific stuff in there behind a static key instead of
changing these definitions?

Or did I miss a subtle user somewhere else?

Will