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Re: [PATCH 4/5] multiboot2: parse console= option when setting GOP mode
- To: Roger Pau Monné <roger.pau@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Thu, 30 Mar 2023 10:52:21 +0200
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- Cc: marmarek@xxxxxxxxxxxxxxxxxxxxxx, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx, Daniel Kiper <daniel.kiper@xxxxxxxxxx>
- Delivery-date: Thu, 30 Mar 2023 08:52:32 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 30.03.2023 10:11, Roger Pau Monné wrote:
> On Thu, Mar 30, 2023 at 08:24:20AM +0200, Jan Beulich wrote:
>> On 29.03.2023 18:29, Roger Pau Monné wrote:
>>> On Mon, Dec 05, 2022 at 04:10:28PM +0100, Jan Beulich wrote:
>>>> On 23.11.2022 16:45, Roger Pau Monne wrote:
>>>>> @@ -265,6 +266,15 @@ __efi64_mb2_start:
>>>>> cmpl $MULTIBOOT2_TAG_TYPE_END,MB2_tag_type(%rcx)
>>>>> je .Lrun_bs
>>>>>
>>>>> + /*
>>>>> + * Get command line from Multiboot2 information.
>>>>> + * Must be last parsed tag.
>>>>
>>>> Why? And how do you guarantee this?
>>>
>>> I think the comment is misleading, must be the last checked for tag in
>>> the loop that does this in assembly, because it's not using cmove.
>>> I've adjusted to:
>>>
>>> /* Get command line from Multiboot2 information. */
>>> cmpl $MULTIBOOT2_TAG_TYPE_CMDLINE,MB2_tag_type(%rcx)
>>> jne .Lno_cmdline
>>> lea MB2_tag_string(%rcx),%rdx
>>> jmp .Lefi_mb2_next_tag
>>> .Lno_cmdline:
>>>
>>> Maybe there's some instruction I'm missing similar to a conditional
>>> lea?
>>
>> There isn't. If you want to get away without conditional branch, then
>> you'll need to LEA to a scratch register (unconditional) and then
>> CMOVcc from that scratch register.
>
> Likely not worth it, unless you dislike the extra conditional branch.
Entirely up to you - I'm fine either way.
Jan
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