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    SubjectRe: [RFC 3/5] x86: Add a comment clarifying LDT context switching
    On Mon, Oct 20, 2014 at 10:41 PM, Borislav Petkov <bp@alien8.de> wrote:
    > On Thu, Oct 16, 2014 at 09:21:42AM -0700, Andy Lutomirski wrote:
    >> I think it's the same as in the other case in switch_mm. leave_mm does
    >> cpumask_clear_cpu(cpu, mm_cpumask(active_mm)), and, once that has
    >> happened, modify_ldt won't send an IPI to this CPU. So, if leave_mm
    >> runs, and then another CPU calls modify_ldt on the mm that is in lazy
    >> mode here, it won't update our LDT register, so the LDT register and
    >> prev->context.ldt might not match.
    >
    > Ok, let me see if I can follow with an example:
    >
    > We call leave_mm() on, say, cpu 3 and mm_cpumask(active_mm) has cpu 3 and
    > 4 set. Then, on cpu 4 we call modify_ldt on that same mm and there in
    > alloc_ldt() we have this:
    >
    > if (!cpumask_equal(mm_cpumask(current->mm),
    > cpumask_of(smp_processor_id())))
    > smp_call_function(flush_ldt, current->mm, 1);
    >
    > and since we've cleared cpu 3 from the cpumask, we don't flush_ldt()
    > on it and there you have the difference.
    >
    > Am I close?

    You're exactly correct, or at least you seem to understand it the way I do :)

    --Andy


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