Date: Fri, 8 Jun 2012 01:43:01 +0300 From: Konstantin Belousov <kostikbel@gmail.com> To: Peter Wemm <peter@wemm.org> Cc: Gianni <gianni@freebsd.org>, Alan Cox <alc@rice.edu>, Alexander Kabaev <kan@freebsd.org>, Attilio Rao <attilio@freebsd.org>, freebsd-arch@freebsd.org, Dag-Erling Sm?rgrav <des@des.no> Subject: Re: Fast vs slow syscalls (Re: Fwd: [RFC] Kernel shared variables) Message-ID: <20120607224301.GB85127@deviant.kiev.zoral.com.ua> In-Reply-To: <CAGE5yCp0VUwcPh1_L2uU=wmCh96pkrrpuZWNMNw6RuMnYPyXQw@mail.gmail.com> References: <CACfq090r1tWhuDkxdSZ24fwafbVKU0yduu1yV2%2BoYo%2BwwT4ipA@mail.gmail.com> <201206051008.29568.jhb@freebsd.org> <86haupvk4a.fsf@ds4.des.no> <201206051222.12627.jhb@freebsd.org> <20120605171446.GA28387@onelab2.iet.unipi.it> <20120606040931.F1050@besplex.bde.org> <864nqovoek.fsf@ds4.des.no> <20120607064951.C1106@besplex.bde.org> <86sje7sf31.fsf@ds4.des.no> <CAGE5yCp0VUwcPh1_L2uU=wmCh96pkrrpuZWNMNw6RuMnYPyXQw@mail.gmail.com>
next in thread | previous in thread | raw e-mail | index | archive | help
--ds9maZbwT7uk2FVi Content-Type: text/plain; charset=koi8-r Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Thu, Jun 07, 2012 at 03:30:54PM -0700, Peter Wemm wrote: > On Thu, Jun 7, 2012 at 1:26 AM, Dag-Erling Sm?rgrav <des@des.no> wrote: > > Bruce Evans <brde@optusnet.com.au> writes: > >> Now 2.44 nsec/call makes sense, but you really should add some volatil= es > >> here to ensure that getpid() is not optimized away. > > > > As you can see from the disassembly I provided, it isn't. > > > >> SO it loops OK, but we can't see what getpid() does. =9AIt must not be > >> doing much. > > > > Umm, yes, that's the whole point of this conversation. =9ALinux's getpi= d() > > is not a syscall, but a library function that returns a constant from a > > page shared by the kernel. >=20 > It might be worth taking a peek at what they do before going too far > down the rabbit hole. They've had to deal with the whole ABI > stability vs kernel layout thing already. >=20 > As I recall, they literally embed a userland style .so shared object > into the kernel and make it available to the user. The dynamic linker > "finds" it via elf auxinfo and inserts it into the symbol search > order. >=20 > That way, the shared page layout is kernel specific. If they chose to > provide getpid() or gettimeofday() or whatever, its a matter of > adjusting the shared page and inserting code into the .so file. If > the page changes, the code changes. >=20 > Think of what we do with signal trampolines except in a way > ld-elf.so.1 can pull it into user space and gdb "sees" it as a .so > file with debug info. >=20 > I think I remember that they did the shared page thing and then > switched to providing a stub .so file. Yes, this is the thing called VDSO in the thread discussion. --ds9maZbwT7uk2FVi Content-Type: application/pgp-signature Content-Disposition: inline -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.12 (FreeBSD) iEYEARECAAYFAk/RLnUACgkQC3+MBN1Mb4hULgCg6R/ekHO3tW9BYjjiMafdKXmR gccAoLWFdYgh2qDFvMB7fGpWn1myusKE =HShI -----END PGP SIGNATURE----- --ds9maZbwT7uk2FVi--
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?20120607224301.GB85127>