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    SubjectRe: Question about x86/mm/gup.c's use of disabled interrupts
    Jeremy Fitzhardinge wrote:
    > Avi Kivity wrote:
    >>> Hm, awkward if flush_tlb_others doesn't IPI...
    >>>
    >>
    >> How can it avoid flushing the tlb on cpu [01]? It's it's
    >> gup_fast()ing a pte, it may as well load it into the tlb.
    >
    > xen_flush_tlb_others uses a hypercall rather than an IPI, so none of
    > the logic which depends on there being an IPI will work.

    Right, of course, that's what we were talking about. I thought
    optimizations to avoid IPIs if an mm never visited a cpu.

    >
    >>> Simplest fix is to make gup_get_pte() a pvop, but that does seem
    >>> like putting a red flag in front of an inner-loop hotspot, or
    >>> something...
    >>>
    >>> The per-cpu tlb-flush exclusion flag might really be the way to go.
    >>
    >> I don't see how it will work, without changing Xen to look at the flag?
    >>
    >> local_irq_disable() is used here to lock out a remote cpu, I don't
    >> see why deferring the flush helps.
    >
    > Well, no, not deferring. Making xen_flush_tlb_others() spin waiting
    > for "doing_gup" to clear on the target cpu. Or add an explicit notion
    > of a "pte update barrier" rather than implicitly relying on the tlb
    > IPI (which is extremely convenient when available...).

    Pick up a percpu flag from all cpus and spin on each? Nasty.

    You could use the irq enabled flag; it's available and what native spins
    on (but also means I'll need to add one if I implement this).

    --
    I have a truly marvellous patch that fixes the bug which this
    signature is too narrow to contain.



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