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 <[email protected]> > Signed-off-by: Dapeng Mi <[email protected]> > Signed-off-by: Zide Chen <[email protected]> > --- > 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,
