Re: [PATCH] target/arm: Provide always-false kvm_arm_*_supported() stubs for usermode

[Pierrick Bouvier <pierrick.bouvier@linaro.org>]

On 7/14/25 6:51 AM, Peter Maydell wrote:
> If you try to build aarch64-linux-user with clang and --enable-debug then it
> fails to compile:
> 
>   ld: libqemu-aarch64-linux-user.a.p/target_arm_cpu64.c.o: in function `cpu_arm_set_sve':
>   ../../target/arm/cpu64.c:321:(.text+0x1254): undefined reference to `kvm_arm_sve_supported'
> 
> This is a regression introduced in commit f86d4220, which switched
> the kvm-stub.c file away from being built for all arm targets to only
> being built for system emulation binaries.  It doesn't affect gcc,
> presumably because even at -O0 gcc folds away the always-false
> kvm_enabled() condition but clang does not.
> 
> We would prefer not to build kvm-stub.c once for usermode and once
> for system-emulation binaries, and we can't build it just once for
> both because it includes cpu.h.  So instead provide always-false
> versions of the five functions that are valid to call without KVM
> support in kvm_arm.h.
> 
> Fixes: f86d42205c2eba ("target/arm/meson: accelerator files are not needed in user mode")
> Resolves: https://gitlab.com/qemu-project/qemu/-/issues/3033
> Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
> ---
> I'm never sure when we prefer to use stub-functions in separate C files
> vs when we prefer to have ifdeffed stubs in headers. There are several
> ways we could fix this compile error, so I just picked one...
> ---
>   target/arm/kvm_arm.h | 35 +++++++++++++++++++++++++++++++++++
>   1 file changed, 35 insertions(+)
>

Thanks Peter, clang with --enable-debug is indeed a combination I didn't 
try. I'll test this too now. Going through this topic, yes I noticed 
that gcc always folds the any if (0) condition, and, based on a Richard 
comment (I can't find now) it seems that we used to rely on that for 
other parts of the code.

The fix you propose works well (initial goal was just to remove 
CONFIG_KVM, so having CONFIG_USER_ONLY is ok), but I wonder if there is 
something specific affecting clang in this case and preventing the folding.

Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>

Thanks,
Pierrick