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    SubjectRe: [kvm-devel] [PATCH] KVM: Avoid using vmx instruction directly
    H. Peter Anvin wrote:
    > Jeremy Fitzhardinge wrote:
    >> Avi Kivity wrote:
    >>>> Or gcc
    >>>> might move the assignment of phys_addr to after the inline assembly.
    >>> "asm volatile" prevents that (and I'm not 100% sure it's necessary).
    >> No, it won't necessarily. "asm volatile" simply forces gcc to emit the
    >> assembler, even if it thinks its output doesn't get used. It makes no
    >> ordering guarantees with respect to other code (or even other "asm
    >> volatiles"). The "memory" clobbers should fix the ordering of the asms
    >> though.
    > I think you're wrong about that; in particular, I'm pretty sure "asm
    > volatiles" are ordered among themselves. What the "volatile" means is
    > "this has side effects you (the compiler) don't understand", and gcc
    > can't assume that it can reorder such side effects.

    The gcc manual has this to say:

    Similarly, you can't expect a sequence of volatile `asm' instructions
    to remain perfectly consecutive. If you want consecutive output, use a
    single `asm'. Also, GCC will perform some optimizations across a
    volatile `asm' instruction; GCC does not "forget everything" when it
    encounters a volatile `asm' instruction the way some other compilers do.

    I wonder how we are supposed to code the following sequence:

    asm volatile ("blah") /* sets funky processor mode */


    asm volatile ("unblah");

    Let's say "blah" disables floating point exceptions, and some_c_code()
    must run without exceptions. Is is possible to code this in gcc without
    putting functions in another translation unit? Is a memory clobber
    sufficient? I'd certainly hate to use it.

    Do not meddle in the internals of kernels, for they are subtle and quick to panic.

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      Last update: 2006-11-21 20:45    [W:0.021 / U:5.140 seconds]
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