Re: [PATCH v2 1/4] ArmPkg/CpuDxe ARM: avoid splitting page table sections unnecessarily

[Leif Lindholm <leif.lindholm@linaro.org>]

On Thu, Mar 02, 2017 at 10:36:13AM +0000, Ard Biesheuvel wrote:
> Currently, any range passed to CpuArchProtocol::SetMemoryAttributes is
> fully broken down into page mappings if the start or the size of the
> region happens to be misaliged relative to the section size of 1 MB.
> 
> This is going to hurt when we enable strict memory permissions, given

"Hurt" -> "cause unnecessary performance penalties" or "hurt" ->
"break everything"?

> that we remap the entire RAM space non-executable (modulo the code
> bits) when the CpuArchProtocol is installed.
> 
> So refactor the code to iterate over the range in a way that ensures
> that all naturally aligned section sized subregions are not broken up.
> 
> Contributed-under: TianoCore Contribution Agreement 1.0
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>

Many thanks for getting rid of the magic values, and in general making
the code more logical. One question below:

> ---
>  ArmPkg/Drivers/CpuDxe/Arm/Mmu.c | 47 ++++++++++++++++----
>  1 file changed, 39 insertions(+), 8 deletions(-)
> 
> diff --git a/ArmPkg/Drivers/CpuDxe/Arm/Mmu.c b/ArmPkg/Drivers/CpuDxe/Arm/Mmu.c
> index 89e429925ba9..ce4d529bda67 100644
> --- a/ArmPkg/Drivers/CpuDxe/Arm/Mmu.c
> +++ b/ArmPkg/Drivers/CpuDxe/Arm/Mmu.c
> @@ -679,6 +679,7 @@ SetMemoryAttributes (
>    )
>  {
>    EFI_STATUS    Status;
> +  UINT64        ChunkLength;
>  
>    //
>    // Ignore invocations that only modify permission bits
> @@ -687,14 +688,44 @@ SetMemoryAttributes (
>      return EFI_SUCCESS;
>    }
>  
> -  if(((BaseAddress & 0xFFFFF) == 0) && ((Length & 0xFFFFF) == 0)) {
> -    // Is the base and length a multiple of 1 MB?
> -    DEBUG ((EFI_D_PAGE, "SetMemoryAttributes(): MMU section 0x%x length 0x%x to %lx\n", (UINTN)BaseAddress, (UINTN)Length, Attributes));
> -    Status = UpdateSectionEntries (BaseAddress, Length, Attributes, VirtualMask);
> -  } else {
> -    // Base and/or length is not a multiple of 1 MB
> -    DEBUG ((EFI_D_PAGE, "SetMemoryAttributes(): MMU page 0x%x length 0x%x to %lx\n", (UINTN)BaseAddress, (UINTN)Length, Attributes));
> -    Status = UpdatePageEntries (BaseAddress, Length, Attributes, VirtualMask);
> +  while (Length > 0) {

Would this not end up returning an uninitialized Status if called with
Length == 0?

/
    Leif

> +    if ((BaseAddress % TT_DESCRIPTOR_SECTION_SIZE == 0) &&
> +        Length >= TT_DESCRIPTOR_SECTION_SIZE) {
> +
> +      ChunkLength = Length - Length % TT_DESCRIPTOR_SECTION_SIZE;
> +
> +      DEBUG ((DEBUG_PAGE,
> +        "SetMemoryAttributes(): MMU section 0x%lx length 0x%lx to %lx\n",
> +        BaseAddress, ChunkLength, Attributes));
> +
> +      Status = UpdateSectionEntries (BaseAddress, ChunkLength, Attributes,
> +                 VirtualMask);
> +    } else {
> +
> +      //
> +      // Process page by page until the next section boundary, but only if
> +      // we have more than a section's worth of area to deal with after that.
> +      //
> +      ChunkLength = TT_DESCRIPTOR_SECTION_SIZE -
> +                    (BaseAddress % TT_DESCRIPTOR_SECTION_SIZE);
> +      if (ChunkLength + TT_DESCRIPTOR_SECTION_SIZE > Length) {
> +        ChunkLength = Length;
> +      }
> +
> +      DEBUG ((DEBUG_PAGE,
> +        "SetMemoryAttributes(): MMU page 0x%lx length 0x%lx to %lx\n",
> +        BaseAddress, ChunkLength, Attributes));
> +
> +      Status = UpdatePageEntries (BaseAddress, ChunkLength, Attributes,
> +                 VirtualMask);
> +    }
> +
> +    if (EFI_ERROR (Status)) {
> +      break;
> +    }
> +
> +    BaseAddress += ChunkLength;
> +    Length -= ChunkLength;
>    }
>  
>    // Flush d-cache so descriptors make it back to uncached memory for subsequent table walks
> -- 
> 2.7.4
>