f9f5ab5718
The virNumaNodeIsAvailable function is stubbed out when building
without libnuma, such that it just returns a constant value. When
CLang is optimizing, it does inter-procedural analysis across
function calls. When it sees that the call to virNumaNodeIsAvailable
returns a fixed constant, it elides the conditional check for errors
in the callers such as virNumaNodesetIsAvailable.
This is a valid optimization as the C standard declares that there
must only be one implementation of each function in a binary. This
is normally the case, but ELF allows for function overrides when
linking or at runtime with LD_PRELOAD, which is technically outside
the mandated C language behaviour.
So while CLang's optimization works fine at runtime, it breaks in our
test suite which aims to mock the virNumaNodeIsAvailable function so
that it has specific semantics regardless of whether libnuma is built
or not. The return value check optimization though means our mock
override won't have the right effect. The mock will be invoked, but
its return value is not used.
Potentially the same problem could be exhibited with GCC if certain
combinations of optimizations are enabled, though thus far we've
not seen it.
To be robust on both CLang and GCC we need to make it more explicit
that we want to be able to replace functions and thus optimization
of calls must be limited. Currently we rely on 'noinline' which
does successfully prevent inlining of the function, but it cannot
stop the eliding of checks based on the constant return value.
Thus we need a bigger hammer.
There are a couple of options to disable this optimization:
* Annotate a symbol as 'weak'. This is tells the compiler
that the symbol is intended to be overridable at linktime
or runtime, and thus it will avoid doing inter-procedural
analysis for optimizations. This was tried previously but
have to be reverted as it had unintended consequences
when linking .a files into our final .so, resulting in all
the weak symbol impls being lost. See commit
407a281a8e2b6c5078ba1148535663ea64fd9314
* Annotate a symbol with 'noipa'. This tells the compiler
to avoid inter-procedural analysis for calls to just this
function. This would be ideal match for our scenario, but
unfortunately it is only implemented for GCC currently:
https://reviews.llvm.org/D101011
* The '-fsemantic-interposition' argument tells the optimizer
that any functions may be replaced with alternative
implementations that have different semantics. It thus
blocks any optimizations across function calls. This is
quite a harsh block on the optimizer, but it appears to be
the only one that is viable with CLang.
Out of those choices option (3) is the only viable option for
CLang. We don't want todo it for GCC though as it is such a
big hammer. Probably we should apply (2) for GCC, should we
experiance a problem in future.
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Libvirt API for virtualization
Libvirt provides a portable, long term stable C API for managing the virtualization technologies provided by many operating systems. It includes support for QEMU, KVM, Xen, LXC, bhyve, Virtuozzo, VMware vCenter and ESX, VMware Desktop, Hyper-V, VirtualBox and the POWER Hypervisor.
For some of these hypervisors, it provides a stateful management daemon which runs on the virtualization host allowing access to the API both by non-privileged local users and remote users.
Layered packages provide bindings of the libvirt C API into other languages including Python, Perl, PHP, Go, Java, OCaml, as well as mappings into object systems such as GObject, CIM and SNMP.
Further information about the libvirt project can be found on the website:
License
The libvirt C API is distributed under the terms of GNU Lesser General Public License, version 2.1 (or later). Some parts of the code that are not part of the C library may have the more restrictive GNU General Public License, version 2.0 (or later). See the files COPYING.LESSER and COPYING for full license terms & conditions.
Installation
Instructions on building and installing libvirt can be found on the website:
https://libvirt.org/compiling.html
Contributing
The libvirt project welcomes contributions in many ways. For most components the best way to contribute is to send patches to the primary development mailing list. Further guidance on this can be found on the website:
https://libvirt.org/contribute.html
Contact
The libvirt project has two primary mailing lists:
- libvirt-users@redhat.com (for user discussions)
- libvir-list@redhat.com (for development only)
Further details on contacting the project are available on the website: