Re: [PATCH v3] hw/i386: fix short-circuit logic with non-optimizing builds

[Philippe Mathieu-Daudé]

(Cc'ing Eric)

On 20/11/23 10:28, Michael S. Tsirkin wrote:
> On Sun, Nov 19, 2023 at 07:34:58PM -0600, Dan Hoffman wrote:
> > As far as I can tell, yes. Any optimization level above O0 does not have this
> > issue (on this version of Clang, at least)
>
> Aha, this is with -O0. That makes sense.

But then, why the other cases aren't problematic?

$ git grep -E ' (&&|\|\|) !?kvm_enabled'
hw/arm/boot.c:1228:    assert(!(info->secure_board_setup && kvm_enabled()));
hw/i386/microvm.c:270:        (mms->rtc == ON_OFF_AUTO_AUTO && 
!kvm_enabled())) {
hw/i386/x86.c:135:    if (x86ms->apic_id_limit > 255 && kvm_enabled() &&
hw/mips/cps.c:62:    return is_mt && !kvm_enabled();
system/physmem.c:760:    assert(asidx == 0 || !kvm_enabled());
target/arm/cpu64.c:288:    if (value && kvm_enabled() && 
!kvm_arm_sve_supported()) {
target/i386/cpu.c:7264:    if (requested_lbr_fmt && kvm_enabled()) {
target/ppc/kvm.c:345:    if (!cpu->hash64_opts || !kvm_enabled()) {
target/s390x/cpu_models.c:574:    if (xcc->kvm_required && !kvm_enabled()) {
target/s390x/cpu_models_sysemu.c:124:    if 
(S390_CPU_CLASS(oc)->kvm_required && !kvm_enabled()) {

> We have:
>    ;;
>    --enable-debug)
>        # Enable debugging options that aren't excessively noisy
>        meson_option_parse --enable-debug-tcg ""
>        meson_option_parse --enable-debug-graph-lock ""
>        meson_option_parse --enable-debug-mutex ""
>        meson_option_add -Doptimization=0
>        default_cflags='-O0 -g'
>
>
> > On Sun, Nov 19, 2023 at 4:54 PM Philippe Mathieu-Daudé <philmd@linaro.org>
> > wrote:
> >
> >      Hi,
> >
> >      On 19/11/23 21:31, Daniel Hoffman wrote:
> >      > `kvm_enabled()` is compiled down to `0` and short-circuit logic is
> >      > used to remove references to undefined symbols at the compile stage.
> >      > Some build configurations with some compilers don't attempt to
> >      > simplify this logic down in some cases (the pattern appears to be
> >      > that the literal false must be the first term) and this was causing
> >      > some builds to emit references to undefined symbols.
> >      >
> >      > An example of such a configuration is clang 16.0.6 with the following
> >      > configure: ./configure --enable-debug --without-default-features
> >      > --target-list=x86_64-softmmu --enable-tcg-interpreter
> >
> >      Is the '--enable-debug' option triggering this?
> >
> >      I'm surprised the order of conditions matters for code elision...
> >
> >      > Signed-off-by: Daniel Hoffman <dhoff749@gmail.com>
> >      > ---
> >      >   hw/i386/x86.c | 15 ++++++++++++---
> >      >   1 file changed, 12 insertions(+), 3 deletions(-)
> >      >
> >      > diff --git a/hw/i386/x86.c b/hw/i386/x86.c
> >      > index b3d054889bb..2b6291ad8d5 100644
> >      > --- a/hw/i386/x86.c
> >      > +++ b/hw/i386/x86.c
> >      > @@ -131,8 +131,12 @@ void x86_cpus_init(X86MachineState *x86ms, int
> >      default_cpu_version)
> >      >       /*
> >      >        * Can we support APIC ID 255 or higher?  With KVM, that requires
> >      >        * both in-kernel lapic and X2APIC userspace API.
> >      > +     *
> >      > +     * kvm_enabled() must go first to ensure that kvm_* references are
> >      > +     * not emitted for the linker to consume (kvm_enabled() is
> >      > +     * a literal `0` in configurations where kvm_* aren't defined)
> >      >        */
> >      > -    if (x86ms->apic_id_limit > 255 && kvm_enabled() &&
> >      > +    if (kvm_enabled() && x86ms->apic_id_limit > 255 &&
> >      >           (!kvm_irqchip_in_kernel() || !kvm_enable_x2apic())) {
> >      >           error_report("current -smp configuration requires kernel "
> >      >                        "irqchip and X2APIC API support.");
> >      > @@ -418,8 +422,13 @@ void x86_cpu_pre_plug(HotplugHandler *hotplug_dev,
> >      >       }
> >      >       cpu->thread_id = topo_ids.smt_id;
> >      >
> >      > -    if (hyperv_feat_enabled(cpu, HYPERV_FEAT_VPINDEX) &&
> >      > -        kvm_enabled() && !kvm_hv_vpindex_settable()) {
> >      > +    /*
> >      > +    * kvm_enabled() must go first to ensure that kvm_* references are
> >      > +    * not emitted for the linker to consume (kvm_enabled() is
> >      > +    * a literal `0` in configurations where kvm_* aren't defined)
> >      > +    */
> >      > +    if (kvm_enabled() && hyperv_feat_enabled(cpu, HYPERV_FEAT_VPINDEX) &
> >      &
> >      > +        !kvm_hv_vpindex_settable()) {
> >      >           error_setg(errp, "kernel doesn't allow setting HyperV
> >      VP_INDEX");
> >      >           return;
> >      >       }