Re: RFC x86/boot/64: BOOT_PGT_SIZE definition for compressed kernel

[Arvind Sankar <nivedita@alum.mit.edu>]

On Tue, Oct 27, 2020 at 03:40:07PM +0300, Kirill A. Shutemov wrote:
> On Sat, Oct 24, 2020 at 08:41:58PM -0400, Arvind Sankar wrote:
> > Hi, I think the definition of BOOT_PGT_SIZE in
> > arch/x86/include/asm/boot.h is insufficient, especially after
> >   ca0e22d4f011 ("x86/boot/compressed/64: Always switch to own page table")
> > 
> > Currently, it allocates 6 pages if KASLR is disabled, and either 17 or
> > 19 pages depending on X86_VERBOSE_BOOTUP if KASLR is enabled.
> > 
> > - The X86_VERBOSE_BOOTUP test shouldn't be done: that only disables
> >   debug messages, but warnings/errors are always output to VGA memory,
> >   so the two extra pages for mapping video RAM are always needed.
> > 
> > - The calculation wasn't updated for X86_5LEVEL, which requires at least
> >   one more page for the P4D level, and in theory could require two extra
> >   pages for each of the 4 mappings (compressed kernel, output kernel,
> >   boot_params and command line), though that would require a system with
> >   truly ginormous amounts of RAM.
> 
> Or sparse physical memory map. I hacked QEMU before for testing 5-level
> paging:
> 
> https://gist.github.com/kiryl/d45eb54110944ff95e544972d8bdac1d
> 
> > - If KASLR is disabled, there are only 6 pages, but now that we're
> >   always setting up our own page table, we need 1+(2+2)*3 (one PGD, and
> >   two PUD and two PMD pages for kernel, boot_params and command line),
> >   and 2 more pages for the video RAM, and more for 5-level. Even for
> >   !RELOCATABLE, 13 pages might be needed.
> 
> The comment for BOOT_PGT_SIZE has to be updated.
> 
> BTW, what happens if we underestimate BOOT_PGT_SIZE? Do we overwrite
> something?

No, it checks whether it ran out of pages, so it will just error out and
hang.

> 
> > - SEV-ES needs one more page because it needs to do a PTE-level mapping
> >   for the GHCB page.
> > 
> > - The static calculation is also busted because
> >   boot/compressed/{kaslr.c,acpi.c} can scan the setup data, EFI
> >   configuration tables and the EFI memmap, and none of these are
> >   accounted for. They used to be scanned while still on the
> >   firmware/bootloader page tables, but now our page tables have to cover
> >   them as well. Trying to add up the worst case for all of these, and
> >   anything else the compressed kernel might potentially access seems
> >   like a lost cause.
> > 
> > We could do something similar to what the main kernel does with
> > early_dynamic_pgts: map the compressed kernel at a fixed virtual
> > address (in negative address space, say); recycle all the other mappings
> > until we're done with decompression, and then map the output,
> > boot_params and command line. The number of pages needed for this can be
> > statically calculated, for 4-level paging we'd need 2 pages for the
> > fixed mapping, 12 pages for the other three, and one PGD page.
> 
> Recycling idea look promising to me, but it would require handling #PF in
> decompression code, right? It is considerable complication of the code.
> 

The #PF handler is already there now with the SEV-ES series, but I agree
it would still complicate things. It's simpler to just increase
BOOT_PGT_SIZE and make it unconditional (i.e. bump it to say 32 or 64
even if !KASLR). It's @nobits anyway so it would not increase the size
of the bzImage, just require a slightly larger memory allocation by the
bootloader.

Another alternative is reusing the KASLR code, which contains a memory
allocator, and use it to find system memory for the page tables, but
that also seems like an over-engineered approach.