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

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

On Mon, Mar 06, 2017 at 03:55:42PM +0100, Ard Biesheuvel wrote:
> On 6 March 2017 at 15:12, Leif Lindholm <leif.lindholm@linaro.org> wrote:
> > 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"?
> >
> 
> The former. It will map all of RAM using page mappings, which uses
> more space unnecessarily

OK, can you expand the statement to say "hurt performance" then?

> >> 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?
> >
> 
> Yes, well spotted. I will just add an early 'return EFI_SUCCESS' for this case.

Works for me. If you also tweak the commit message:
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>

> >> +    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
> >>