On 15/09/2020 14:46, Leif Lindholm wrote:
On Tue, Sep 15, 2020 at 14:14:30 +0100, Grant Likely wrote:
The EFI stub in Linux removes /memreserve/ entries from the DT before
handing it to the kernel proper.
commit 0ceac9e094b065fe3fec19669740f338d3480498
Author: Mark Salter <[email protected]>
Date: Mon Sep 8 13:01:08 2014 -0400
efi/arm64: Fix fdt-related memory reservation
Does that still make sense? I understand why it was done, but is it
right to ignore those reservations outright?
Yes. It is duplication of (sources of) information, forcing the
operating system to make runtime, or compile time, judgement calls of
which source(s) of information to respect.
Not quite that simple. We're not talking about a clean cut-over from
non-UEFI to UEFI platforms, but rather a phased transition where with a
given DT, both the non-UEFI and UEFI boot paths need to work. e.g.,
U-Boot platforms where most people are using 'bootm', but want to start
encouraging them to use the UEFI infrastructure.
Or in other words, the master source of information is the .dts file,
not the firmware itself.
The other issue is that the reserved memory region may not be about
firmware at all, but rather a memory layout that is wanted only by the
OS. Regardless of the approach we take here, those regions must be
respected.
As more U-Boot platforms
turn on UEFI there could be unexpected consequences if the memory
reservation block are silently ignored. I'm think that on the U-Boot
platforms it is more likely that /memreserve/ is in use.
That should also make it easy to intercept? Like putting a hook in the
DT update code that triggers build error/warning (or even update the
UEFI memory map) if someone is trying to memreserve with the UEFI
interface enabled.
It should not be an error to use /memreserve/. That creates a hard break
between UEFI and non-UEFI boot paths for /memreserve/. Updating the
memory map is fine, which leads to the question of what memory type
should be used?
EfiBootServicesData: Memory still gets mapped in the linear map, but
nothing protects it after ExitBootServices (would require leaving
/memreserve/ intact so the OS knows to protect them).
EfiReservedMemory: (As I understand it) Doesn't need /memreserve/, but
causes a change in behaviour. The memory will not appear in the linear
map. This will possibly cause problems with existing drivers
EfiRuntimeServicesData: Keeps the region protected and in the linear
map, but feels 'wrong'. An OS might decide to reclaim it anyway if it
doesn't use runtime services (against spec?).
Other options?
It should be fine for /memreserve/ entries to get applied to the memmap
during boot. Are there problems that I'm missing?
Sure. They can be applied in the UEFI memory map. By u-boot, during
boot.
/
Leif
g.
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