Function efi_add_known_memory uses the configured DRAM banks (gd->bd->bi_dram) to define the memory that an EFI application may use.
For qemu-x86_defconfig this will result in the first 1 MB of physical memory being available. Here we find the BIOS, interrupt vectors and the VGA memory (0xA0000-0xBFFFF).
As a result grubia32.efi writes unknowingly to the video memory.
For reference: In function install_e820_map() we block [ISA_START_ADDRESS, ISA_END_ADDRESS[.
The problem seems to stem from file arch/x86/cpu/qemu/dram.c, function dram_init_banksize():
gd->bd->bi_dram[0].start = 0; gd->bd->bi_dram[0].size = gd->ram_size;
Probably exluding [ISA_START_ADDRESS, ISA_END_ADDRESS[ is not sufficient as this does not protect interrupt vectors.
Could you, please, provide reasonable values.
Maybe a better idea would be to define reserved memory like the ones that we find in the device trees, e.g.
reserved-memory { #address-cells = <2>; #size-cells = <2>; ranges;
/* 16 MiB reserved for Hardware ROM Firmware */ hwrom_reserved: hwrom@0 { reg = <0x0 0x0 0x0 0x1000000>; no-map; };
But U-Boot does not yet support the concept of reserved memory.
Best regards
Heinrich