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    SubjectRe: [patch 134/149] x86, paravirt: Add a global synchronization point for pvclock
    On 07/14/2010 10:45 AM, Zachary Amsden wrote:
    > On 07/14/2010 10:40 AM, Jeremy Fitzhardinge wrote:
    >> On 07/14/2010 01:16 PM, Avi Kivity wrote:
    >>> On 07/14/2010 08:57 PM, Jeremy Fitzhardinge wrote:
    >>>> Anything else?
    >>> 1. set up a mapping
    >>> 2. invlpg or set cr3
    >>> 3. use the mapping
    >>>
    >>> Moving the invlpg will break your code.
    >> invlpg uses memory clobbers. All the crX ops seem to use a
    >> __force_order variable to sequence them - but it looks like it's done
    >> precisely backwards and it's barking mad to do allow write_crX to be
    >> reordered with respect to memory ops.
    >>
    >> Hm, looks like glommer added it surreptitiously while unifying
    >> system_32.h and system_64.h (system_32.h relied on asm volatile not
    >> being reordered; system_64.h used memory clobbers).
    >> J
    >
    > clts() has no memory clobber; it is used to serialize execution of
    > code within kernel_fpu_begin() / kernel_fpu_end() blocks.
    >
    > If the code within is reordered before the clts(), we've corrupted
    > guest FPU state.
    >
    > That's the kind of bug I think Linus is talking about. We've been
    > expecting volatile to work that way for over a decade, by my
    > recollection, and if it doesn't, there is going to be a lot of broken
    > code.
    >
    > Shouldn't we at least get a compiler switch to force the volatile
    > behavior? I'd suggest it default to conservative.

    Hmm, well, despite that not being quite correct (if guest has used FPU,
    we save it, which has a memory clobber), it seems to be the case that a
    reordering of the clts() among the other volatile asm statements would
    be a very bad thing - you'd get kernel FPU exceptions.

    And among asm volatiles, clts() is fairly unique in not having any
    clobbers or dependencies at all, so it could happen.

    Zach


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