On December 28, 2022 6:31:07 PM PST, "Jason A. Donenfeld" <ja...@zx2c4.com> wrote: >Hi, > >Read this message in a fixed width text editor with a lot of columns. > >On Wed, Dec 28, 2022 at 03:58:12PM -0800, H. Peter Anvin wrote: >> Glad you asked. >> >> So the kernel load addresses are parameterized in the kernel image >> setup header. One of the things that are so parameterized are the size >> and possible realignment of the kernel image in memory. >> >> I'm very confused where you are getting the 64 MB number from. There >> should not be any such limitation. > >Currently, QEMU appends it to the kernel image, not to the initramfs as >you suggest below. So, that winds up looking, currently, like: > > kernel image setup_data > |--------------------------||----------------| >0x100000 0x100000+l1 0x100000+l1+l2 > >The problem is that this decompresses to 0x1000000 (one more zero). So >if l1 is > (0x1000000-0x100000), then this winds up looking like: > > kernel image setup_data > |--------------------------||----------------| >0x100000 0x100000+l1 0x100000+l1+l2 > > d e c o m p r e s s e d k e r n e l > > |-------------------------------------------------------------| > 0x1000000 > 0x1000000+l3 > >The decompressed kernel seemingly overwriting the compressed kernel >image isn't a problem, because that gets relocated to a higher address >early on in the boot process. setup_data, however, stays in the same >place, since those links are self referential and nothing fixes them up. >So the decompressed kernel clobbers it. > >The solution in this commit adds a bunch of padding between the kernel >image and setup_data to avoid this. That looks like this: > > kernel image padding > setup_data > > |--------------------------||---------------------------------------------------||----------------| >0x100000 0x100000+l1 >0x1000000+l3 0x1000000+l3+l2 > > d e c o m p r e s s e d k e r n e l > > |-------------------------------------------------------------| > 0x1000000 > 0x1000000+l3 > >This way, the decompressed kernel doesn't clobber setup_data. > >The problem is that if 0x1000000+l3-0x100000 is around 62 megabytes, >then the bootloader crashes when trying to dereference setup_data's >->len param at the end of initialize_identity_maps() in ident_map_64.c. >I don't know why it does this. If I could remove the 62 megabyte >restriction, then I could keep with this technique and all would be >well. > >> In general, setup_data should be able to go anywhere the initrd can >> go, and so is subject to the same address cap (896 MB for old kernels, >> 4 GB on newer ones; this address too is enumerated in the header.) > >It would be theoretically possible to attach it to the initrd image >instead of to the kernel image. As a last resort, I guess I can look >into doing that. However, that's going to require some serious rework >and plumbing of a lot of different components. So if I can make it work >as is, that'd be ideal. However, I need to figure out this weird 62 meg >limitation. > >Any ideas on that? > >Jason
As far as a crash... that sounds like a big and a pretty serious one at that. Could you let me know what kernel you are using and how *exactly* you are booting it?
