Hello,
My recent boot pagetable changes have caused me to work with the EFI
build of Xen rather more than previously.
First, there is a dependency tracking bug in the build system. Edits to
xen/arch/x86/efi/efi-boot.h don't cause xen.efi to be regenerated. From
what I can tell, the file doesn't even get recompiled, because syntax
errors even go unnoticed.
Second, and the main point of the email.
The EFI code has some logic which does:
if ( !base_video )
{
...
if ( best != StdOut->Mode->Mode )
StdOut->SetMode(StdOut, best);
}
just before printing out the Xen banner. This has a side effect of
causing all further use of StdOut/StdErr to cease working, and
interfering completely with debugging activities. (Waiting for a
keypress on StdIn however does work, which is how I eventually diagnosed
that it was an output problem.) Skipping this logic allows debugging to
work.
The code appeared in bf6501a62 "x86-64: EFI boot code" and has no
specific description of what it is doing and/or trying to achieve.
It is also not entirely clear why it is gated on having a cfg file in
the first place (c/s ,c38cf865ec8, not that there is adequate context
for why) or why there is a Xen command line argument "-basevideo"
introduced in the beginning to skip this behaviour.
As a point of reference, I don't see Linux making any SetMode calls.
What is the purpose of changing to a different mode? Certainly as far
as serial consoles go, sticking with the mode the loader uses certainly
feels like a safer option.
~Andrew
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