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    SubjectRe: [patch 134/149] x86, paravirt: Add a global synchronization point for pvclock
    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.

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