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cpu/x86/64bit/pt: Fix integer arithmethic in assembly

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The GNU assembler allows to use arithmetics on constant expression,
except for division using the '/' character. By default the '/'
introduces a line comment, see [1]. This behaviour can be changed when
the command line argument --divide is being used. However that's not
the case for coreboot.

Since the divide doesn't work as expected the assembler generates 512
times as much instructions on the .rept evaluation than it should. This
didn't cause any problem since it only filled PML4E, but the additional
entries pointed to non existing PDPTs. As long as the memory above 512GB
wasn't accessed it worked just fine.

Use shifts to fix the arithmetic and thus generate only the expected
number of page table entries. Required for the following patch, which
walks page tables and expects to find sane directories.

1: https://sourceware.org/binutils/docs-2.26/as/i386_002dChars.html#i386_002dChars

TEST=GAS generates 512 times less entries on .rept evaluation
Change-Id: I480142df010bf4e7d6fb84c9891e93b6ee21e908
Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/87356
Reviewed-by: Matt DeVillier <matt.devillier@gmail.com>
Reviewed-by: Jérémy Compostella <jeremy.compostella@intel.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
This commit is contained in:
Patrick Rudolph 2025-04-17 15:32:53 +02:00 • committed by Matt DeVillier
commit 094f75162f
2 changed files with 3 additions and 3 deletions
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4
src/cpu/x86/64bit/pt.S
View file

@ -21,14 +21,14 @@
.align 4096
PML4E:
/* For every 512GiB generate a pointer to the corresponding PDPT */
.rept (CONFIG_CPU_PT_ROM_MAP_GB + 511) / 512
.rept ((CONFIG_CPU_PT_ROM_MAP_GB + 511) >> 9)
.quad _GEN_DIR(PDPT + 4096 * ((. - PML4E) >> 3)) /* Point to PDPT */
.endr
.align 4096
PDT:
/* For every 2MiB generate a page entry. In one GiB there are 512 pages. */
.rept 512 * CONFIG_CPU_PT_ROM_MAP_GB
.rept (CONFIG_CPU_PT_ROM_MAP_GB << 9)
.quad _GEN_PAGE(0x200000 * ((. - PDT) >> 3)) /* identity map 2MiB page */
.endr

2
src/cpu/x86/64bit/pt1G.S
View file

@ -21,7 +21,7 @@
.align 4096
PML4E:
/* For every 512GiB generate a pointer to the corresponding PDPT */
.rept (CONFIG_CPU_PT_ROM_MAP_GB + 511) / 512
.rept ((CONFIG_CPU_PT_ROM_MAP_GB + 511) >> 9)
.quad _GEN_DIR(PDPT + 4096 * ((. - PML4E) >> 3)) /* Point to PDPT */
.endr