Re: [PATCH v2 4/7] srcu: Add irqsave variants of the atomic SRCU readers

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

On 2026-11-11 19:07, Paul E. McKenney wrote:
[...]
> Although that doesn't help on my laptop, it does work on my x86 test
> system.

Is the laptop a hybrid CPU, with P-cores and E-cores?  KVM gives its
guests no PMU on those.  kvm_init_pmu_capability() in arch/x86/kvm/pmu.c
clears enable_pmu under X86_FEATURE_HYBRID_CPU.

> Perhaps something like the following would be helpful in keeping this
> well-tested?

Yes, that would help.  Three comments on the patch:

[...]
> +             qemu_args='--qemu-args "-cpu host,pmu=on"'
[...]
> +             --configs "SRCU-P" ${qemu_args} --trust-make

1) The quotes inside the variable stay literal.  The expansion is split
    on spaces, so kvm.sh gets three words:

        --qemu-args
        "-cpu
        host,pmu=on"

    A kvm.sh --dryrun with these arguments stops with:

        --qemu-args (qemu arguments) ""-cpu" must match "^-"

    Keeping only the value in the variable works:

        qemu_arg="-cpu host,pmu=on"
        [...]
        --configs "SRCU-P" ${qemu_arg:+--qemu-args "$qemu_arg"} --trust-make

    It expands to two words when set, and to nothing when empty.  I
    checked it with kvm.sh --dryrun only.

2) nmi_readers and nmi_calls both default to true.  So the two boot
    arguments change nothing.  They are fine if you want them explicit.

3) The "atomic-srcu" run just above also runs readers in NMI, by
    default.  It still starts without a PMU, so it keeps printing the
    "went untested" lines.  It could take the same qemu argument.

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com