Re: [dpdk-dev] [PATCH 6/6] doc: guide for Windows build using MinGW-w64

[Thomas Monjalon <thomas@monjalon.net>]

Hi,

We didn't get a confirmation from Microsoft about the TLS problem.
Please could we have a status?

Should we mark shared library linkage as not supported in Windows DPDK?
Is there a difference between clang and MinGW?


17/02/2020 07:27, Dmitry Kozlyuk:
> > Remembered another issue: thread-local storage (TLS) with shared libraries.
> > Windows PE doesn't support TLS via special sections, so compilers use TLS
> > emulation layer. With static libraries, there are no issues described below.
> > 
> > The first aspect is a build-time issue of MinGW. When linking to DPDK shared
> > libraries, errors occur:
> > 
> > 	undefined reference to `__emutls_v.per_lcore__rte_errno'
> > 	undefined reference to `__emutls_v.per_lcore__rte_lcore_id'
> > 
> > DPDK declares per_lcore__XXX in a map file, but GCC places __thread symbols
> > in __emutls_v section, so the proper name to export becomes __emutls_v.XXX.
> > This can be worked around by using an additional version script with MinGW,
> > as I do in my port [0], however, the proper solution would be fixing the bug
> > on MinGW side [1]. MinGW already converts TLS variable names when generating
> > DEF files with `-Wl,--output-def` option (not used by DPDK, just a hint).
> 
> Did some research and AFAICT, there is not effortless solution for
> efficient per-lcore variables on Windows. While MinGW-w64 has aforementioned
> issues (actually, GCC on Windows does), Clang with default TLS options just
> generates wrong results when exporting variables from dynamic libraries.
> Demo: https://github.com/PlushBeaver/tlstest
> 
> Thread [0] claims this is a fundamental problem with PE-COFF executable
> format, but I honestly lack expertise to tell if this is valid. Microsoft
> docs [1] suggests that exporting __thread variables won't just work. Can
> someone from Microsoft or from UNH Lab comment further?
> 
> [0]: https://sourceforge.net/p/mingw-w64/mailman/message/31777672/
> [1]:
> https://docs.microsoft.com/en-us/windows/win32/dlls/using-thread-local-storage-in-a-dynamic-link-library