> On Dec 28, 2019, at 4:51 PM, Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx> wrote: > > On Sat, 28 Dec 2019 at 08:03, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote: >> >> >> >>>> On Dec 28, 2019, at 2:35 PM, Arvind Sankar <nivedita@xxxxxxxxxxxx> wrote: >>> >>> On Sat, Dec 28, 2019 at 01:29:00PM +0800, Andy Lutomirski wrote: >>>> >>>>> * The stack must be 16-byte aligned >>>> >>>> Nope. The asm needs to do this for runtime services. The kernel runs with 8-byte stack alignment. >>>> >>> 32-bit code is actually only 4-byte aligned in the kernel proper, right? >> >> Right. By “8” I meant “long”. Sorry. >> >>> >>> Currently, only native 64-bit calls always respect the 16-byte alignment >>> requirement, by aligning explicitly in the asm stubs, or after the >>> cleanup patches, via the efi bootloader running with 16-byte stack >>> alignment. >>> >>> I think mixed mode might actually be aligned via the asm stub in the >>> kernel proper, though it doesn't look like it is in the bootloader >>> portion. >> >> The underlying problem is that gcc doesn’t give us a way to do CALL from asm while preserving more than a single word of alignment. This forces us to compile the kernel proper with reduced alignment. (Also, the generated code is better with reduced alignment.) > > At runtime, the 64-bit kernel always uses a 16 byte aligned stack when > calling into EFI (32 or 64 bit), either by aligning the stack pointer, > or by switching to a special stack. Can you point me at the stack switching code? Stack switches always make me nervous due to interactions with other things, especially NMIs.
