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Re: [PATCH 4/5] multiboot2: parse console= option when setting GOP mode
- To: Jan Beulich <jbeulich@xxxxxxxx>
- From: Roger Pau Monné <roger.pau@xxxxxxxxxx>
- Date: Thu, 30 Mar 2023 10:11:50 +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 08:12:14 +0000
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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.
Thanks, Roger.
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