On Mon, Apr 28, 2025 at 03:05:55PM -0700, Dave Hansen wrote: > On 4/10/25 22:37, Changyuan Lyu wrote: > > From: Alexander Graf <g...@amazon.com> > > > > +/* > > + * If KHO is active, only process its scratch areas to ensure we are not > > + * stepping onto preserved memory. > > + */ > > Same thing on the imperative voice here. > > I'm also not fully understanding the comment. Do these "scratch" regions > basically represent all the memory that's not being handed over? It's > not obvious.
Scratch memory represents areas created at the boot of the first kernel and it's known that scratch areas won't contain any memory that's being handed over. > > diff --git a/arch/x86/kernel/e820.c b/arch/x86/kernel/e820.c > > index 57120f0749cc3..c314212a5ecd5 100644 > > --- a/arch/x86/kernel/e820.c > > +++ b/arch/x86/kernel/e820.c > > @@ -1300,6 +1300,24 @@ void __init e820__memblock_setup(void) > > memblock_add(entry->addr, entry->size); > > } > > > > + /* > > + * At this point with KHO we only allocate from scratch memory. > > + * At the same time, we configure memblock to only allow > > + * allocations from memory below ISA_END_ADDRESS which is not > > + * a natural scratch region, because Linux ignores memory below > > + * ISA_END_ADDRESS at runtime. Beside very few (if any) early > > + * allocations, we must allocate real-mode trapoline below > > trampoline ^ > > > + * ISA_END_ADDRESS. > > + * > > + * To make sure that we can actually perform allocations during > > + * this phase, let's mark memory below ISA_END_ADDRESS as scratch > > + * so we can allocate from there in a scratch-only world. > > + * > > + * After real mode trampoline is allocated, we clear scratch > > + * marking from the memory below ISA_END_ADDRESS > > + */ > > + memblock_mark_kho_scratch(0, ISA_END_ADDRESS); > > This isn't making a whole ton of sense to me. > > Is this *only* to facilitate possible users that need <ISA_END_ADDRESS > allocations? If so, please say that. > > I _think_ this is trying to say that KHO kernels are special and are > trying to only allocate from scratch areas. But <ISA_END_ADDRESS > allocations are both necessary and not marked by KHO _as_ a scratch area > which causes a problem. Yes :) -- Sincerely yours, Mike.
