Re: [PATCH v2 4/4] x86/64/mm: Map all kernel memory into trampoline_pgd
From: Dave Hansen
Date: Wed Sep 29 2021 - 11:22:37 EST
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;
> 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 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 = trampoline_pgd_entry.pgd;
> - trampoline_pgd = init_top_pgt.pgd;
Nit: can we preserve the order, please?
/* Map the real mode stub as virtual == physical */
trampoline_pgd = trampoline_pgd_entry.pgd;
for (i = pgd_index(__PAGE_OFFSET); i < PTRS_PER_PGD; i++)
trampoline_pgd[i] = init_top_pgt[i].pgd;