On 9/29/21 7:55 AM, Joerg Roedel wrote: ... > The reason this does not crash always is only that kernel mappings are > global and the CR3 switch does not flush those mappings. But if theses > mappings are not in the TLB already, the above code will crash before it > can jump to the real-mode stub. This would have been nice to have in the cover letter. The whole purpose for this series wasn't totally apparent until I read this. > diff --git a/arch/x86/realmode/init.c b/arch/x86/realmode/init.c > index 0cfe1046cec9..792cb9ca9b29 100644 > --- a/arch/x86/realmode/init.c > +++ b/arch/x86/realmode/init.c > @@ -91,6 +91,7 @@ static void __init setup_real_mode(void) > #ifdef CONFIG_X86_64 > u64 *trampoline_pgd; > u64 efer; > + int i; > #endif > > base = (unsigned char *)real_mode_header; > @@ -147,8 +148,17 @@ static void __init setup_real_mode(void) > trampoline_header->flags = 0; > > trampoline_pgd = (u64 *) __va(real_mode_header->trampoline_pgd); > + > + /* > + * Map all of kernel memory into the trampoline PGD so that it includes > + * the direct mapping and vmalloc space. This is needed to keep the > + * stack and real_mode_header mapped when switching to this page table. > + */ This comment's mention of the direct map and vmalloc() makes a lot of sense in the context of this patch where you're adding them. But, it doesn't mention the pgd[511] stuff. Maybe just make it more generic: Include the entirety of the kernel mapping into the trampoline PGD. This way, all mappings present in the normal kernel page tables are usable while running on trampoline_pgd. > + for (i = pgd_index(__PAGE_OFFSET); i < PTRS_PER_PGD; i++) > + trampoline_pgd[i] = init_top_pgt[i].pgd; > + > + /* Map the real mode stub as virtual == physical */ > trampoline_pgd[0] = trampoline_pgd_entry.pgd; > - trampoline_pgd[511] = init_top_pgt[511].pgd; > #endif Nit: can we preserve the order, please? /* Map the real mode stub as virtual == physical */ trampoline_pgd[0] = trampoline_pgd_entry.pgd; for (i = pgd_index(__PAGE_OFFSET); i < PTRS_PER_PGD; i++) trampoline_pgd[i] = init_top_pgt[i].pgd;
