Re: [PATCH] i386/kvm: Fix kvm_enable_x2apic link error in non-KVM builds

["Philippe Mathieu-Daudé" <philmd@linaro.org>]

On 28/11/24 20:06, Phil Dennis-Jordan wrote:
> On Thu, 28 Nov 2024 at 17:46, Philippe Mathieu-Daudé <philmd@linaro.org 
> <mailto:philmd@linaro.org>> wrote:
> 
>     On 28/11/24 17:38, Phil Dennis-Jordan wrote:
>      > Paolo, could we please apply either Sairaj and Santosh's fix at
>      > https://patchew.org/QEMU/20241114114509.15350-1-sarunkod@amd.com/
>     <https://patchew.org/QEMU/20241114114509.15350-1-sarunkod@amd.com/>
>      >
>     <https://patchew.org/QEMU/20241114114509.15350-1-sarunkod@amd.com/
>     <https://patchew.org/QEMU/20241114114509.15350-1-sarunkod@amd.com/>>
>      > or mine to fix this link error? As neither patch has so far been
>     merged,
>      > 9.2.0-rc2 still fails to build on macOS, at least on my local
>     systems.
>      > I'm not sure why CI builds aren't jumping up and down about this,
>     but
>      > neither the Xcode 15.2 nor 16.1 toolchains are happy on macOS
>     14.7/arm64.
> 
>     Just curious, is your build configured with --enable-hvf --enable-tcg?
> 
> 
> It's my understanding that both HVF and TCG are enabled by default when 
> building on macOS - they both show up as "YES" in the ./configure 
> output, and the relevant -accel works; at any rate, specifying them 
> explicitly made no difference with regard to this link error. Your 
> question did however prompt me to dig a little deeper and check which of 
> my test configurations was affected.
> 
> It looks like the critical setting is --enable-debug. I think that 
> changes the exact optimisation level (not -O0 but less aggressive than 
> the default), so it's not unreasonable that this would change the 
> compiler pass(es) for eliminating constant conditional branches.
> 
> So yeah, when I build latest master/staging with --enable-debug on macOS 
> and my --target-list includes x86_64, QEMU fails to link with an 
> undefined symbol error for _kvm_enable_x2apic. This happens on both 
> arm64 and x86-64 hosts, and with various Xcode 15.x and 16.y toolchains.

Indeed:

C compiler for the host machine: clang (clang 16.0.0 "Apple clang 
version 16.0.0 (clang-1600.0.26.4)")
C linker for the host machine: clang ld64 1115.7.3
Host machine cpu family: aarch64
Host machine cpu: aarch64
   Compilation
     host CPU                        : aarch64
     host endianness                 : little
     C compiler                      : clang
     Host C compiler                 : clang
     C++ compiler                    : NO
     Objective-C compiler            : clang
     Rust support                    : NO
     CFLAGS                          : -g -O0
   User defined options
     optimization                    : 0

Undefined symbols for architecture arm64:
   "_kvm_enable_x2apic", referenced from:
       _amdvi_sysbus_realize in hw_i386_amd_iommu.c.o
ld: symbol(s) not found for architecture arm64


> I have to admit I'm personally not a big fan of relying on the optimiser 
> for removing references to these symbols, but restructuring the 
> conditional expression like in Sairaj and Santosh's patch seems to allow 
> even the optimisation level used for debug builds to do it, so I guess I 
> can't argue with the result. :-)

See related commit 9926cf34de5 ("target/i386: Allow elision of 
kvm_enable_x2apic()").