Re: [PATCH v4 06/15] arm64: Make the PHYS_MASK_SHIFT dynamic

Will Deacon
2024-07-09T11:43:44Z

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@arm.com>
> Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
> Signed-off-by: Steven Price <steven.price@arm.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