Messages in this thread | | | Date | Thu, 02 Mar 2023 07:06:11 -0800 | From | "Andy Lutomirski" <> | Subject | Re: [tip: x86/urgent] x86/setup: Always reserve the first 1M of RAM |
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On Thu, Mar 2, 2023, at 2:50 AM, Borislav Petkov wrote: > On Wed, Mar 01, 2023 at 07:51:43PM -0800, Andy Lutomirski wrote: >> This is quite broken. The comments in the patch seem to understand >> that Linux tries twice to allocate the real mode trampoline, but the >> code has some issues. >> >> First, it actively breaks the logic here: >> >> + /* >> + * Don't free memory under 1M for two reasons: >> + * - BIOS might clobber it >> + * - Crash kernel needs it to be reserved >> + */ >> + if (start + size < SZ_1M) >> + continue; >> + if (start < SZ_1M) { >> + size -= (SZ_1M - start); >> + start = SZ_1M; >> + } >> + > > Are you refering, per-chance, here to your comment in that same function > a bit higher? > > Introduced by this thing here: > > 5bc653b73182 ("x86/efi: Allocate a trampoline if needed in > efi_free_boot_services()") > > ?
Yes.
> > Also, it looks like Mike did pay attention to your commit: > > https://lore.kernel.org/all/YLZsEaimyAe0x6b3@kernel.org/
He definitely did. But I'm still pretty sure the patch in question broke it :-/
> > And then there's the whole deal with kdump kernel needing lowmem. The > function which became obsolete and got removed by: > > 23721c8e92f7 ("x86/crash: Remove crash_reserve_low_1M()") > > So, considering how yours is the only report that breaks booting and > this reservation of <=1M has been out there for ~2 years without any > complaints, I'm thinking what we should do now is fix that logic. > > Btw, this whole effort started with > > a799c2bd29d1 ("x86/setup: Consolidate early memory reservations") > > Also see this: > > ec35d1d93bf8 ("x86/setup: Document that Windows reserves the first MiB") > > and with shit like that, we're "piggybacking" on Windoze since there > certification happens at least. > > Which begs the question: how does your laptop even boot on windoze if > windoze reserves that 1M too?!
I haven't booted Windoze on this thing in years. But...
There is no possible way that Windoze genuinely reserves the first 1M. It does SMP, and x86 needs <1M memory for SMP, so Windoze uses <1M memory. QED :)
> >> I real the commit message and the linked bug, and I'm having trouble >> finding evidence of anything actually fixed by this patch. Can we >> just revert it? If not, it would be nice to get a fixup patch that >> genuinely cleans this up -- the whole structure of the code (first, >> try to allocate trampoline, then free boot services, then try again) >> isn't really conducive to a model where we *don't* free boot services >> < 1M. > > Yes, I think this makes most sense. And that whole area is a minefield > so the less we upset the current universe, the better.
I'll send a revert patch.
Thinking about this a bit more, if we actually want to "reserve" <1M, we should implement it completely differently by treating <1M as its very own special thing and teaching the memblock allocator to refuse to allocate <1M unless specifically requested. There's only a very small number of allocations that need it (crashkernel for some reason?), and there are at least two spurious users of memblock_phys_alloc_range that curently may use <1M but have no business doing so (ramdisk code and the NUMA distance table). But let's only do that if there's an actual problem to solve.
> >> Discovered by my delightful laptop, which does not boot with this patch applied. > > How come your laptop hasn't booted new Linux since then?!? Tztztztz
Honestly, no clue. Looking at the logs, I'm pretty sure I *did* boot an affected (6.0) kernel. The actual problematic memory map on this laptop seems to show up a bit inconsistently as some horrible combination of firmware settings (especially SGX) and who-knows-what else. My best guess is that a GRUB update I installed yesterday caused some tiny memory map change that triggered it.
I did install a new kernel yesterday too, but the *previous* kernel stopped booting too.
> > Thx. > > -- > Regards/Gruss, > Boris. > > https://people.kernel.org/tglx/notes-about-netiquette
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