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SubjectRe: [crash, bisected] Re: [PATCH 3/4] x86_64: Fold pda into per cpu area
Mike Travis wrote:
> Oh yeah, is it alright to re-use the pda in the static percpu load area
> for each startup cpu, or should it be adjusted to use the areas allocated
> by setup_per_cpu_areas()? pda_init() is called in x86_64_start_kernel
> so it would only be for anything that occurs before then. (And I moved
> the call to pda_init() to before the early_idt_handlers are setup.)
>

Why not use the real pda for all cpus?

Do you move the boot-cpu's per-cpu data? (Please don't) If not, you can
just use percpu__pda from the start without having to do anything else,
and then set up %gs pointing to the pda base for each secondary cpu.

64-bit inherits 32-bit's use of per-cpu gdts, though its mostly useless
on 64-bit.

More important is to have a:

startup_percpu_base: .quad __per_cpu_load

which you stick the processor's initial %gs into, and then load that
from in startup_secondary_64:

mov $X86_MSR_GSBASE, %ecx
mov startup_percpu_base, %eax
mov startup_percpu_base+4, %edx
wrmsr

and put

startup_percpu_base = new_cpus_percpu_base;

in do_cpu_boot().

> I hadn't realized that this code is executed for cpus other than the
> boot cpu. Is there a way to find out if this is the boot cpu (and/or
> the initial execution)?
>

Don't think so. If you want something to happen only at boot time, do
it in startup_64.

> If it's the boot cpu, then this would work for the gdt, yes?
>
> leaq early_gdt_descr_base(%rip), %edi
> movq 0(%edi), %rax
> addq $__per_cpu_load, %rax
> movq %rax, 0(%edi)
> lgdt early_gdt_descr(%rip)
>

As I mentioned in my other mail, a simple add should be enough.

> But it should only be executed for the boot because do_boot_cpu()
> does this:
>
> early_gdt_descr.address = (unsigned long)get_cpu_gdt_table(cpu);
>
> static inline struct desc_struct *get_cpu_gdt_table(unsigned int cpu)
> {
> return per_cpu(gdt_page, cpu).gdt;
> }
>

Right, do it in startup_64.

> Btw, I've only been testing on an x86_64 system. I'm sure I've got
> things to fix up for i386.
>

It should be possible to share almost everything, at least in C.

J


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