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;