Re: [RFC v2-fix 1/1] x86/boot: Add a trampoline for APs booting in 64-bit mode

[Dan Williams]

On Wed, May 19, 2021 at 5:19 PM Kuppuswamy, Sathyanarayanan
<sathyanarayanan.kuppuswamy@xxxxxxxxxxxxxxx> wrote:
>
> Hi Dan,
>
> On 5/17/21 9:08 PM, Dan Williams wrote:
> >> SYM_DATA_START_LOCAL(tr_idt)
> >>           .short  0
> >>           .quad   0
> >> SYM_DATA_END(tr_idt)
> > This format implies that tr_idt is reserving space for 2 distinct data
> > structure attributes of those sizes, can you just put those names here
> > as comments? Otherwise the .fill format is more compact.
>
> Initially its 6 bytes (2 bytes for IDT limit, 4 bytes for 32 bit linear
> start address). This patch extends it by another 4 bytes for supporting
> 64 bit mode.
>
> 2 bytes IDT limit (.short)
> 8 bytes for 64 bit IDT start address (.quad)
>
> This info is included in commit log. But I will add comment here as you
> have mentioned.

Thanks. I only read commit logs when code comments fail.

>
> Will following comment log do ?
>
> /* Use 10 bytes for IDT (in 64 bit mode), 8 bytes for IDT start address
>     2 bytes for IDT limit size */

I would clarify how the boot code uses this:

"When a bootloader hands off to the kernel in 32-bit mode an IDT with
a 2-byte limit and 4-byte base is needed. When a boot loader hands off
to a kernel 64-bit mode the base address extends to 8-bytes. Reserve
enough space for either scenario."