Re: [kvm-unit-tests PATCH v3 22/27] arm64: Use code from the gnu-efi when booting with EFI

Ricardo Koller
2022-07-01T00:43:59Z

On Thu, Jun 30, 2022 at 11:03:19AM +0100, Nikos Nikoleris wrote:
> arm/efi/crt0-efi-aarch64.S defines the header and the handover
> sequence from EFI to a efi_main. This change includes the whole file
> in arm/cstart64.S when we compile with EFI support.
>
> In addition, we change the handover code in arm/efi/crt0-efi-aarch64.S
> to align the stack pointer. This alignment is necessary because we
> make assumptions about cpu0's stack alignment and most importantly we
> place its thread_info at the bottom of this stack.
>
> Signed-off-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
> ---
> arm/cstart64.S | 6 ++++++
> arm/efi/crt0-efi-aarch64.S | 21 +++++++++++++++++----
> 2 files changed, 23 insertions(+), 4 deletions(-)
>
> diff --git a/arm/cstart64.S b/arm/cstart64.S
> index 55b41ea..08cf02f 100644
> --- a/arm/cstart64.S
> +++ b/arm/cstart64.S
> @@ -15,6 +15,10 @@
> #include <asm/thread_info.h>
> #include <asm/sysreg.h>
>
> +#ifdef CONFIG_EFI
> +#include "efi/crt0-efi-aarch64.S"
> +#else
> +
> .macro zero_range, tmp1, tmp2
> 9998: cmp \tmp1, \tmp2
> b.eq 9997f
> @@ -107,6 +111,8 @@ start:
> bl exit
> b halt
>
> +#endif
> +
> .text
>
> /*
> diff --git a/arm/efi/crt0-efi-aarch64.S b/arm/efi/crt0-efi-aarch64.S
> index d50e78d..03d29b0 100644
> --- a/arm/efi/crt0-efi-aarch64.S
> +++ b/arm/efi/crt0-efi-aarch64.S
> @@ -111,10 +111,19 @@ section_table:
>
> .align 12
> _start:
> - stp x29, x30, [sp, #-32]!
> + stp x29, x30, [sp, #-16]!
> +
> + /* Align sp; this is necessary due to way we store cpu0's thread_info */
> mov x29, sp
> + and x29, x29, #THREAD_MASK
> + mov x30, sp
> + mov sp, x29
> + str x30, [sp, #-16]!
> +
> + mov x29, sp

I wasn't sure what was this x29 for. But after some googling, this is
what I found [0]:

The frame pointer (X29) should point to the previous frame pointer saved
on stack, with the saved LR (X30) stored after it.

The old code ended up with x29 pointing to the right place: the previous
(x29,x30).

| ... |
| x1 |
| x0 |
| x30 |
x29 -> | x29 |

In the new code x29 is pointing to:

| ... |
| x30 |
old_sp->| x29 |
| ... |
| x1 |
| x0 |
| pad |
x29 -> | old_sp |

I think the new version can be fixed by setting x29 to the old_sp,
conveniently stored in x30:

+ mov x30, sp

> +
> + stp x0, x1, [sp, #-16]!
>
> - stp x0, x1, [sp, #16]
> mov x2, x0
> mov x3, x1
> adr x0, ImageBase
> @@ -123,8 +132,12 @@ _start:
> bl _relocate
> cbnz x0, 0f
>
> - ldp x0, x1, [sp, #16]
> + ldp x0, x1, [sp], #16
> bl efi_main
>
> -0: ldp x29, x30, [sp], #32
> + /* Restore sp */
> + ldr x30, [sp], #16
> + mov sp, x30
> +
> +0: ldp x29, x30, [sp], #16
> ret
> --
> 2.25.1
>

[0] https://developer.arm.com/documentation/den0024/a/The-ABI-for-ARM-64-bit-Architecture/Register-use-in-the-AArch64-Procedure-Call-Standard/Indirect-result-location