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SubjectRe: [PATCH 1/3] x86/setup: always reserve the first 1M of RAM
On Tue, Jun 01, 2021 at 10:53:52AM +0300, Mike Rapoport wrote:
> From: Mike Rapoport <>
> 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 <>
> ---
> 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.



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