On Mon, Mar 13, 2023 at 18:16:38 +0100, Ard Biesheuvel wrote:
> With large page support out of the picture, we can treat bits 1 and 0 of
> the page descriptor as individual valid and XN bits, instead of treating
> XN as a page type. Doing so aligns the handling of the attribute with
> the section descriptor layout, as well as the XN handling on AArch64,
> and this is beneficial for maintainability.
On average, this is a big enough improvement that I'm happy for it to
go in, but it *does* take a step away from the actual architectural
definition.
I'll choose to see it as viewing aarch32 through aarch64 goggles,
which feels reasonable these days.
Reviewed-by: Leif Lindholm <quic_llindhol@quicinc.com>
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> ---
> ArmPkg/Include/Chipset/ArmV7Mmu.h | 8 +++-----
> ArmPkg/Library/ArmMmuLib/Arm/ArmMmuLibUpdate.c | 12 ++++++------
> 2 files changed, 9 insertions(+), 11 deletions(-)
>
> diff --git a/ArmPkg/Include/Chipset/ArmV7Mmu.h b/ArmPkg/Include/Chipset/ArmV7Mmu.h
> index 7501ebfdf97f..6a2584ceb303 100644
> --- a/ArmPkg/Include/Chipset/ArmV7Mmu.h
> +++ b/ArmPkg/Include/Chipset/ArmV7Mmu.h
> @@ -54,11 +54,9 @@
> #define TT_DESCRIPTOR_SECTION_TYPE_IS_PAGE_TABLE(Desc) (((Desc) & 3UL) == TT_DESCRIPTOR_SECTION_TYPE_PAGE_TABLE)
>
> // Translation table descriptor types
> -#define TT_DESCRIPTOR_PAGE_TYPE_MASK (3UL << 0)
> -#define TT_DESCRIPTOR_PAGE_TYPE_FAULT (0UL << 0)
> -#define TT_DESCRIPTOR_PAGE_TYPE_PAGE (2UL << 0)
> -#define TT_DESCRIPTOR_PAGE_TYPE_PAGE_XN (3UL << 0)
> -#define TT_DESCRIPTOR_PAGE_TYPE_LARGEPAGE (1UL << 0)
> +#define TT_DESCRIPTOR_PAGE_TYPE_MASK (1UL << 1)
> +#define TT_DESCRIPTOR_PAGE_TYPE_FAULT (0UL << 1)
> +#define TT_DESCRIPTOR_PAGE_TYPE_PAGE (1UL << 1)
>
> // Section descriptor definitions
> #define TT_DESCRIPTOR_SECTION_SIZE (0x00100000)
> diff --git a/ArmPkg/Library/ArmMmuLib/Arm/ArmMmuLibUpdate.c b/ArmPkg/Library/ArmMmuLib/Arm/ArmMmuLibUpdate.c
> index 9ca00c976d5f..12d0f4c30f8e 100644
> --- a/ArmPkg/Library/ArmMmuLib/Arm/ArmMmuLibUpdate.c
> +++ b/ArmPkg/Library/ArmMmuLib/Arm/ArmMmuLibUpdate.c
> @@ -104,12 +104,8 @@ UpdatePageEntries (
>
> // EntryMask: bitmask of values to change (1 = change this value, 0 = leave alone)
> // EntryValue: values at bit positions specified by EntryMask
> - EntryMask = TT_DESCRIPTOR_PAGE_TYPE_MASK | TT_DESCRIPTOR_PAGE_AP_MASK;
> - if ((Attributes & EFI_MEMORY_XP) != 0) {
> - EntryValue = TT_DESCRIPTOR_PAGE_TYPE_PAGE_XN;
> - } else {
> - EntryValue = TT_DESCRIPTOR_PAGE_TYPE_PAGE;
> - }
> + EntryMask = TT_DESCRIPTOR_PAGE_TYPE_MASK | TT_DESCRIPTOR_PAGE_AP_MASK | TT_DESCRIPTOR_PAGE_XN_MASK;
> + EntryValue = TT_DESCRIPTOR_PAGE_TYPE_PAGE;
>
> // Although the PI spec is unclear on this, the GCD guarantees that only
> // one Attribute bit is set at a time, so the order of the conditionals below
> @@ -148,6 +144,10 @@ UpdatePageEntries (
> EntryValue |= TT_DESCRIPTOR_PAGE_AP_RW_RW;
> }
>
> + if ((Attributes & EFI_MEMORY_XP) != 0) {
> + EntryValue |= TT_DESCRIPTOR_PAGE_XN_MASK;
> + }
> +
> // Obtain page table base
> FirstLevelTable = (ARM_FIRST_LEVEL_DESCRIPTOR *)ArmGetTTBR0BaseAddress ();
>
> --
> 2.39.2
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