Messages in this thread | | | From | Arvind Sankar <> | Date | Tue, 27 Oct 2020 09:16:17 -0400 | Subject | Re: RFC x86/boot/64: BOOT_PGT_SIZE definition for compressed kernel |
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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.
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