Re: [Xen-devel] [PATCH 2/2] build/clang: fix XSM dummy policy when using clang 4.0

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 06.03.17 at 13:31, <roger.pau@xxxxxxxxxx> wrote:
> There seems to be some weird bug in clang 4.0 that prevents xsm_pmu_op from
> working as expected, and vpmu.o ends up with a reference to
> __xsm_action_mismatch_detected which makes the build fail:
> 
> [...]
> ld    -melf_x86_64_fbsd  -T xen.lds -N prelink.o  \
>     xen/common/symbols-dummy.o -o xen/.xen-syms.0
> prelink.o: In function `xsm_default_action':
> xen/include/xsm/dummy.h:80: undefined reference to 
> `__xsm_action_mismatch_detected'
> xen/xen/include/xsm/dummy.h:80: relocation truncated to fit: R_X86_64_PC32 
> against undefined symbol `__xsm_action_mismatch_detected'
> ld: xen/xen/.xen-syms.0: hidden symbol `__xsm_action_mismatch_detected' 
> isn't defined
> 
> Then doing a search in the objects files:
> 
> # find xen/ -type f -name '*.o' -print0 | xargs -0 bash -c \
>   'for filename; do nm "$filename" | \
>   grep -q __xsm_action_mismatch_detected && echo "$filename"; done' bash
> xen/arch/x86/prelink.o
> xen/arch/x86/cpu/vpmu.o
> xen/arch/x86/cpu/built_in.o
> xen/arch/x86/built_in.o
> 
> The current patch is the only way I've found to fix this so far, by simply
> moving the XSM_PRIV check into the default case in xsm_pmu_op. This also fixes
> the behavior of do_xenpmu_op, which will now return -EINVAL for unknown
> XENPMU_* operations, instead of -EPERM when called by a privileged domain.

Especially from this perspective I think the patch is fine (but also
Cc-ing Boris), yet I still think the compilation aspect needs to be got
to the bottom of, to have a complete picture in case the problem
shows up in a slightly different way elsewhere. Did you report this
to clang folks? Did they have any explanation of what's going on?

Jan


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