Re: [PATCH 6/7] target/i386: Make some PEBS features user-visible

["Mi, Dapeng" <dapeng1.mi@linux.intel.com>]

On 1/17/2026 9:10 AM, Zide Chen wrote:
> Populate selected PEBS feature names in FEAT_PERF_CAPABILITIES to make
> the corresponding bits user-visible CPU feature knobs, allowing them to
> be explicitly enabled or disabled via -cpu +/-<feature>.
>
> Once named, these bits become part of the guest CPU configuration
> contract.  If a VM is configured with such a feature enabled, migration
> to a destination that does not support the feature may fail, as the
> destination cannot honor the guest-visible CPU model.
>
> The PEBS_FMT bits are intentionally not exposed. They are not meaningful
> as user-visible features, and QEMU registers CPU features as boolean
> QOM properties, which makes them unsuitable for representing and
> checking numeric capabilities.

Currently KVM supports user space sets PEBS_FMT (see vmx_set_msr()), but
just requires the guest PEBS_FMT is identical with host PEBS_FMT.

IIRC, many places in KVM judges whether guest PEBS is enabled by checking
the guest PEBS_FMT. If we don't expose PEBS_FMT to user space, how does KVM
get the guest PEBS_FMT?


>
> Co-developed-by: Dapeng Mi <dapeng1.mi@linux.intel.com>
> Signed-off-by: Dapeng Mi <dapeng1.mi@linux.intel.com>
> Signed-off-by: Zide Chen <zide.chen@intel.com>
> ---
>  target/i386/cpu.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/target/i386/cpu.c b/target/i386/cpu.c
> index f1ac98970d3e..fc6a64287415 100644
> --- a/target/i386/cpu.c
> +++ b/target/i386/cpu.c
> @@ -1618,10 +1618,10 @@ FeatureWordInfo feature_word_info[FEATURE_WORDS] = {
>          .type = MSR_FEATURE_WORD,
>          .feat_names = {
>              NULL, NULL, NULL, NULL,
> +            NULL, NULL, "pebs-trap", "pebs-arch-reg"
>              NULL, NULL, NULL, NULL,
> -            NULL, NULL, NULL, NULL,
> -            NULL, "full-width-write", NULL, NULL,
> -            NULL, NULL, NULL, NULL,
> +            NULL, "full-width-write", "pebs-baseline", NULL,
> +            NULL, "pebs-timing-info", NULL, NULL,
>              NULL, NULL, NULL, NULL,
>              NULL, NULL, NULL, NULL,
>              NULL, NULL, NULL, NULL,