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SubjectRe: [RFC 0/7] Prep code for better stack switching
On Fri, Nov 10, 2017 at 08:05:19PM -0800, Andy Lutomirski wrote:
> This isn't quite done (the TSS remap patch is busted on 32-bit, but
> that's a straightforward fix), but it should be ready for at least a
> conceptual review.
>
> The idea here is to prepare us to have all kernel data needed for
> user mode execution and early entry located in the fixmap. To do
> this, I hijack the GDT remap mechanism and make it more general. I
> add a struct cpu_entry_area. This struct is never instantiated
> directly. Instead, it represents the layout of a per-cpu portion of
> the fixmap. That portion contains the GDT, the TSS (including IO
> bitmap), and the entry stack (for now just a part of the TSS
> region). It should also end up containing the PEBS and BTS buffers.
>
> If this works, then the idea would be to add a magic *executable* page
> to cpu_entry_area. That page would contain a stub like this:
>
> ENTRY(entry_SYSCALL_64_trampoline)
> UNWIND_HINT_EMPTY
> movq %rsp, 0x1000+entry_SYSCALL_64_trampoline-1f(%rip)
> 1:
> movq 0x1008+entry_SYSCALL_64_trampoline-1f(%rip), %rsp
> 1:
> pushq %rdi
> pushq %rsi

> movq 0x1000+entry_SYSCALL_64_trampoline-1f(%rip), %rsi
> 1:
> movq $entry_SYSCALL_64, %rdi
> jmp *%rdi

So I'm wondering: r12-r15 are callee-preserved so why can't you
scratch into those on entry and leave rsi and rdi pristine so that
entry_SYSCALL_64 can get to work directly?

--
Regards/Gruss,
Boris.

SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg)
--

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