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    SubjectRe: [PATCH v2 08/10] x86-64: Emulate vsyscalls

    * Andrew Lutomirski <luto@mit.edu> wrote:

    > On Mon, May 30, 2011 at 6:59 AM, Ingo Molnar <mingo@elte.hu> wrote:
    > >
    > > * Andrew Lutomirski <luto@mit.edu> wrote:
    > >
    > >>
    > >> I'd advocate waiting until glibc 2.14 comes out with this change:
    > >>
    > >> http://sourceware.org/git/?p=glibc.git;a=commit;h=a8509ca540427502bd955f35296ff7b727c7a8a1
    > >>
    > >> I want to add a warning (ratelimited to an extremely low rate) in v3
    > >> whenever any of the vsyscalls get used telling users that their legacy
    > >> code is suffering a performance impact, but it seems like bad form to
    > >> tell people to build glibc from git to avoid a regression.
    > >
    > > But only statically built binaries would be impacted in practice,
    > > right? The number of statically built binaries that heavily rely on
    > > vsyscalls ought to be a very small set ...
    >
    > With current glibc even dynamic binaries take the hit on time().

    Indeed, you are right, i completely forgot that again :)

    > With the emulation warning (coming in v3), I get (on a Fedora
    > 15-based VM):
    >
    > [ 0.635493] init[1] emulated legacy vsyscall time(); upgrade your
    > code to avoid a performance hit. ip:ffffffffff600404 sp:7fff277fe9c8
    > caller:3da3e9e27d in libc.so.6[3da3e00000+192000]

    Ok, so we should leave the option enabled by default and distros can
    flip it as they upgrade/fix glibc, right?

    Thanks,

    Ingo


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