Re: [PATCH] x86/boot: Don't use INC to set defaults

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 03/10/2024 4:48 pm, Roger Pau Monné wrote:
> On Thu, Oct 03, 2024 at 04:42:23PM +0100, Frediano Ziglio wrote:
>> On Thu, Oct 3, 2024 at 3:58 PM Andrew Cooper <andrew.cooper3@xxxxxxxxxx> 
>> wrote:
>>> __efi64_mb2_start() makes some bold assumptions about the efi_platform and
>>> skip_realmode booleans.  Set them to 1 explicitly, which is more robust.
>>>
>>> Make the comment a little more consice.
>>>
>>> No practical change.
>>>
>>> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>>> ---
>>> CC: Jan Beulich <JBeulich@xxxxxxxx>
>>> CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
>>> CC: Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>
>>> CC: Daniel P. Smith <dpsmith@xxxxxxxxxxxxxxxxxxxx>
>>> CC: Frediano Ziglio <frediano.ziglio@xxxxxxxxx>
>>> ---
>>>  xen/arch/x86/boot/head.S | 10 ++++------
>>>  1 file changed, 4 insertions(+), 6 deletions(-)
>>>
>>> diff --git a/xen/arch/x86/boot/head.S b/xen/arch/x86/boot/head.S
>>> index d1856d8012c9..af776c201a15 100644
>>> --- a/xen/arch/x86/boot/head.S
>>> +++ b/xen/arch/x86/boot/head.S
>>> @@ -279,14 +279,12 @@ __efi64_mb2_start:
>>>          pop     %rbx
>>>          pop     %rax
>>>
>>> -        /* We are on EFI platform and EFI boot services are available. */
>>> -        incb    efi_platform(%rip)
>>> -
>>>          /*
>>> -         * Disable real mode and other legacy stuff which should not
>>> -         * be run on EFI platforms.
>>> +         * efi_multiboot2_prelude() is happy that we're on EFI platform.  
>>> Skip
>>> +         * the BIOS initialisation path.
>>>           */
>>> -        incb    skip_realmode(%rip)
>>> +        movb    $1, efi_platform(%rip)
>>> +        movb    $1, skip_realmode(%rip)
>>>
>>>          /* Jump to trampoline_setup after switching CPU to x86_32 mode. */
>>>          lea     trampoline_setup(%rip),%r15
>>>
>>> base-commit: eb21ce14d709ef0c0030d0625028a4868c81126f
>> Reviewed-by: Frediano Ziglio <frediano.ziglio@xxxxxxxxx>
> Acked-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>


Thanks both.

I further note that efi_platform is used exclusively to skip early
cmdline parsing, and I'm sure there's probably a better way of doing
this, but nothing obvious jumps out to me.

I suspect that getting rid of it will be easier when we've moved more
logic to be in C.

~Andrew