Re: [edk2-devel] [PATCH 1/2] ArmPkg/AsmMacroIoLibV8: Introduce ASM_FUNC_ALIGN()

["Ard Biesheuvel" <ardb@kernel.org>]

On Wed, 19 Apr 2023 at 19:45, Marvin Häuser <mhaeuser@posteo.de> wrote:
>
>
> On 19. Apr 2023, at 19:40, Ard Biesheuvel <ardb@kernel.org> wrote:
>
> On Wed, 19 Apr 2023 at 19:14, Marvin Häuser <mhaeuser@posteo.de> wrote:
>
>
> Hi all,
>
> While testing Ard's suggestion for V3, I noticed I got a broken FD where ArmReplaceLiveTranslationEntry() is misaligned, but does not cross a 4 KB boundary.
>
>
> Which platform are you building?
>
>
> ArmVirtPkg / AARCH64 / DEBUG / GCC5 (GCC 12.2.0).
>
>
> To not just hide the issue via this patch, can someone please try to explain the exact requirements this function has (the comments read like 0x200 was just the lowest value to guarantee staying within a page)? Why would it be broken if misaligned, but not crossing a page?
>
>
> 0x200 is a log2 upper bound for the size of the function, so it's just
> the smallest value that fits that requirement, determined manually
> iirc
>
> And the only reason we have this is that we can cheaply decide whether
> or not unmapping a page will unmap this function or not, but we could
> actually just use the address and size to decide this.
>
> In any case, if the FD is constructed in a way that violates the
> alignment, there is something wrong with the build tools you are
> using.
>
>
> The tools are stock edk2, the only changes made are those in the latest commit of the linked branch.
>
>
> Is there any chance the FD is somehow misaligned in memory, thus shifting the function across a page in the process? Or is the FD mapped to a fixed address like with x86? Is code after ArmReplaceLiveTranslationEntry() crossing page boundaries the actual issue (and is implicitly fixed by aligning it)?
>
>
> If you are building ArmVirtQemu.dsc, the FD is mapped at address 0x0
> and the FV is mapped at 0x1000
>
>
> Then the function simply is not crossing a page boundary... which means the patch did fix a valid bug, but it wasn't what actually caused the corruption. Any help is appreciated. :)
>

Your branch seems to be missing 16e0969ef775b898ac700f3261d76030b8ab9ef0

"ArmVirtPkg/ArmVirtQemu: Use PEI flavor of ArmMmuLib for all PEIMs"