Re: [PATCH V3 11/13] target/i386: Add pebs-fmt CPU option

[Zhao Liu <zhao1.liu@intel.com>]

Hi Zide,

On Wed, Mar 04, 2026 at 10:07:10AM -0800, Zide Chen wrote:
> Date: Wed, 4 Mar 2026 10:07:10 -0800
> From: Zide Chen <zide.chen@intel.com>
> Subject: [PATCH V3 11/13] target/i386: Add pebs-fmt CPU option
> X-Mailer: git-send-email 2.53.0
> 
> Similar to lbr-fmt, target/i386 does not support multi-bit CPU
> properties, so the PEBS record format cannot be exposed as a
> user-visible CPU feature.
> 
> Add a pebs-fmt option to allow users to specify the PEBS format via the
> command line.  Since the PEBS state is part of the vmstate, this option
> is considered migratable.
> 
> We do not support PEBS record format 0.  Although it is a valid format
> on some very old CPUs, it is unlikely to be used in practice.  This
> allows pebs-fmt=0 to be used to explicitly disable PEBS in the case of
> migratable=off.
> 
> If PEBS is not enabled, mark it as unavailable in IA32_MISC_ENABLE and
> clear the PEBS-related bits in IA32_PERF_CAPABILITIES.
> 
> If migratable=on on PEBS capable host and pmu is enabled:
> - PEBS is disabled if pebs-fmt is not specified or pebs-fmt=0.
> - PEBS is enabled if pebs-fmt is set to the same value as the host.
> 
> When migratable=off, the behavior is similar, except that omitting
> the pebs-fmt option does not disable PEBS.
> 
> Signed-off-by: Zide Chen <zide.chen@intel.com>

...

>      if (user_req != -1) {
> +        if (!is_lbr_fmt && !(env->features[FEAT_1_EDX] & CPUID_DTS)) {
> +            error_setg(errp, "vPMU: %s is unsupported without Debug Store", name);
> +            return false;
> +        }
> +

This is a general code path to check user_req != -1... but

> diff --git a/target/i386/kvm/kvm-cpu.c b/target/i386/kvm/kvm-cpu.c
> index 1d0047d037c7..60bf3899852a 100644
> --- a/target/i386/kvm/kvm-cpu.c
> +++ b/target/i386/kvm/kvm-cpu.c
> @@ -231,6 +231,7 @@ static void kvm_cpu_instance_init(CPUState *cs)
>      }
>  
>      cpu->lbr_fmt = -1;
> +    cpu->pebs_fmt = -1;

-1 is only set for KVM, so other accelerators have lbr_fmt=0 by default.

This will cause make check fail:

stderr:
qemu-system-x86_64: vPMU: pebs is unsupported without Debug Store
Broken pipe
../tests/qtest/libqtest.c:201: kill_qemu() tried to terminate QEMU process but encountered exit status 1 (expected 0)

(test program exited with status code -6)

So I think the initialization should be placed in the general code path
as well (x86_cpu_initfn()) - and move lbr_fmt initialization back to
general x86 codes.

Thanks,
Zhao