Re: [PATCH 1/3] efi/x86: simplify 64-bit EFI firmware call wrapper

[Andy Lutomirski <luto@amacapital.net>]

> On Dec 28, 2019, at 4:51 PM, Ard Biesheuvel <ard.biesheuvel@linaro.org> wrote:
> 
> On Sat, 28 Dec 2019 at 08:03, Andy Lutomirski <luto@amacapital.net> wrote:
>> 
>> 
>> 
>>>> On Dec 28, 2019, at 2:35 PM, Arvind Sankar <nivedita@alum.mit.edu> 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.