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Peter Anvin" , Claudio Imbrenda , James Morse , kvm@vger.kernel.org, kvmarm@lists.linux.dev, linux-arm-kernel@lists.infradead.org, linux-s390@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, Marc Zyngier , Ingo Molnar , Nicholas Piggin , Oliver Upton , Suzuki K Poulose , Sven Schnelle , Thomas Gleixner , Will Deacon , x86@kernel.org, Zenghui Yu Subject: Re: Ping? Re: [PATCH rc] kvm: Prevent compiling virt/kvm/vfio.c unless VFIO is selected In-Reply-To: References: <0-v1-08396538817d+13c5-vfio_kvm_kconfig_jgg@nvidia.com> <87edgy87ig.fsf@mail.lhotse> <875y1k3nm9.fsf@mail.lhotse> <20231130011650.GD1389974@nvidia.com> Date: Thu, 30 Nov 2023 17:38:43 +1100 Message-ID: <87y1ef21ak.fsf@mail.lhotse> MIME-Version: 1.0 X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20231129_223856_366774_E2B97034 X-CRM114-Status: GOOD ( 26.26 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org Sean Christopherson writes: > On Wed, Nov 29, 2023, Jason Gunthorpe wrote: >> On Wed, Nov 29, 2023 at 05:07:45PM -0800, Sean Christopherson wrote: >> > On Wed, Nov 29, 2023, Michael Ellerman wrote: >> > > Sean Christopherson writes: >> > > > On Fri, Nov 10, 2023, Michael Ellerman wrote: >> > > >> Jason Gunthorpe writes: >> > > >> > There are a bunch of reported randconfig failures now because of this, >> > > >> > something like: >> > > >> > >> > > >> >>> arch/powerpc/kvm/../../../virt/kvm/vfio.c:89:7: warning: attribute declaration must precede definition [-Wignored-attributes] >> > > >> > fn = symbol_get(vfio_file_iommu_group); >> > > >> > ^ >> > > >> > include/linux/module.h:805:60: note: expanded from macro 'symbol_get' >> > > >> > #define symbol_get(x) ({ extern typeof(x) x __attribute__((weak,visibility("hidden"))); &(x); }) >> > > >> > >> > > >> > It happens because the arch forces KVM_VFIO without knowing if VFIO is >> > > >> > even enabled. >> > > >> >> > > >> This is still breaking some builds. Can we get this fix in please? >> > > >> >> > > >> cheers >> > > >> >> > > >> > Split the kconfig so the arch selects the usual HAVE_KVM_ARCH_VFIO and >> > > >> > then KVM_VFIO is only enabled if the arch wants it and VFIO is turned on. >> > > > >> > > > Heh, so I was trying to figure out why things like vfio_file_set_kvm() aren't >> > > > problematic, i.e. why the existing mess didn't cause failures. I can't repro the >> > > > warning (requires clang-16?), but IIUC the reason only the group code is problematic >> > > > is that vfio.h creates a stub for vfio_file_iommu_group() and thus there's no symbol, >> > > > whereas vfio.h declares vfio_file_set_kvm() unconditionally. >> > > >> > > That warning I'm unsure about. >> > >> > Ah, it's the same warning, I just missed the CONFIG_MODULES=n requirement. >> >> Oh, wait, doesn't that mean the approach won't work? IIRC doesn't >> symbol_get turn into just &fn when non-modular turning this into a >> link failure without the kconfig part? It does build. I haven't boot tested it, but TBH I don't really care as long as the build is green, I don't think anyone's actually using this weird combination of config options. > Yes, but it doesn't cause linker errors. IIUC, because the extern declaration > is tagged "weak", a dummy default is used. E.g. on x86, this is what is generated > with VFIO=y > > fn = symbol_get(vfio_file_is_valid); > if (!fn) > 0xffffffff810396c5 <+5>: mov $0xffffffff81829230,%rax > 0xffffffff810396cc <+12>: test %rax,%rax > > whereas VFIO=n gets > > fn = symbol_get(vfio_file_is_valid); > if (!fn) > 0xffffffff810396c5 <+5>: mov $0x0,%rax > 0xffffffff810396cc <+12>: test %rax,%rax > > I have no idea if the fact that symbol_get() generates '0', i.e. the !NULL checks > work as expected, is intentional or if KVM works by sheer dumb luck. I think it's intentional: https://lore.kernel.org/all/20030117045054.9A2F72C073@lists.samba.org/ cheers _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel