On Tue, Jun 01, 2021 at 10:53:52AM +0300, Mike Rapoport wrote:

From: Mike Rapoport <redacted>

There are BIOSes that are known to corrupt the memory under 1M, or more
precisely under 640K because the memory above 640K is anyway reserved for
the EGA/VGA frame buffer and BIOS.

To prevent usage of the memory that will be potentially clobbered by the
kernel, the beginning of the memory is always reserved. The exact size of
the reserved area is determined by CONFIG_X86_RESERVE_LOW build time and
reservelow command line option. The reserved range may be from 4K to 640K
with the default of 64K. There are also configurations that reserve the
entire 1M range, like machines with SandyBridge graphic devices or systems
that enable crash kernel.

In addition to the potentially clobbered memory, EBDA of unknown size may
be as low as 128K and the memory above that EBDA start is also reserved
early.

It would have been possible to reserve the entire range under 1M unless for
the real mode trampoline that must reside in that area.

To accommodate placement of the real mode trampoline and keep the memory
safe from being clobbered by BIOS reserve the first 64K of RAM before
memory allocations are possible and then, after the real mode trampoline is
allocated, reserve the entire range from 0 to 1M.

Update trim_snb_memory() and reserve_real_mode() to avoid redundant
reservations of the same memory range.

Also make sure the memory under 1M is not getting freed by
efi_free_boot_services().

Fixes: a799c2bd29d1 ("x86/setup: Consolidate early memory reservations")
Signed-off-by: Mike Rapoport <redacted>
---
 arch/x86/kernel/setup.c        | 35 ++++++++++++++++++++--------------
 arch/x86/platform/efi/quirks.c | 12 ++++++++++++
 arch/x86/realmode/init.c       | 14 ++++++++------
 3 files changed, 41 insertions(+), 20 deletions(-)

Ok, let's try it. Booting on a couple of boxes looks ok here, the
difference is visible:

-  DMA zone: 30 pages reserved
+  DMA zone: 159 pages reserved

On the other box, it was already reserving so many pages even before

  DMA zone: 159 pages reserved

i.e., the first 640K.

But it's not like I had problems before with early reservations so my
testing doesn't mean a whole lot. Hugh's testing sounds good, lemme add
his tag too.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette