Messages in this thread | ![/](/images/icornerl.gif) | | Date | Thu, 15 Aug 2019 20:03:02 +0200 (CEST) | From | Thomas Gleixner <> | Subject | Re: [PATCHv6 25/36] vdso: Introduce vdso_static_branch_unlikely() |
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On Thu, 15 Aug 2019, Dmitry Safonov wrote:
> From: Andrei Vagin <avagin@gmail.com> > > As it has been discussed on timens RFC, adding a new conditional branch > `if (inside_time_ns)` on VDSO for all processes is undesirable. > > Addressing those problems, there are two versions of VDSO's .so: > for host tasks (without any penalty) and for processes inside of time > namespace with clk_to_ns() that subtracts offsets from host's time. > > Introduce vdso_static_branch_unlikely(), which is similar to > static_branch_unlikely(); alias it with timens_static_branch_unlikely() > under CONFIG_TIME_NS. > > The timens code in vdso will look like this: > > if (timens_static_branch_unlikely()) { > clk_to_ns(clk, ts);
Please name that clk_to_namespace(). _ns() is widely used for nanoseconds.
> } > > The version of vdso which is compiled from sources will never execute > clk_to_ns(). And then we can patch the 'no-op' in the straight-line > codepath with a 'jump' instruction to the out-of-line true branch and > get the timens version of the vdso library.
Colour me confused. Why do we need that static branch at all?
Why don't we compile VDSO_NO_NAMESPACE and VDSO_NAMESPACE right away? One has the clk_to_namespace() one does not. The you can spare the whole static key patching and the NO_NAMESPACE variant does not have extra 5 NOPS.
The VDSO is one page IIRC, so having the extra namespace variant around does really not matter at all.
Thanks,
tglx
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