Hello,

I have implemented support for exposing and enabling APERF/MPERF
MSR passthrough for x86 QEMU guests when running on KVM.

As of last year, the Linux kernel supports providing a capability
to disable APERF/MPERF read intercepts [1]. However, there is
currently no native way in QEMU to instruct KVM to activate this
capability or expose the feature bit cleanly via CPUID.

My patch introduces the `aperfmperf` feature flag via `FEAT_6_ECX`
(CPUID.06H:ECX[bit 0]). It ties into the existing power management
framework, so when a user enables power management via
`-overcommit cpu-pm=on` and passes `+aperfmperf` to the CPU, QEMU
requests KVM to drop the MSR intercepts.

For testing, I passed the feature via Libvirt XML like so:

    <qemu:arg value="-overcommit"/>
    <qemu:arg value="cpu-pm=on"/>
    <qemu:arg value="-cpu"/>
    <qemu:arg value="host,+aperfmperf,+invtsc"/>

A FreeBSD 16 guest is successfully able to read the changing
values of the MSRs directly without exiting to the hypervisor:

root@freebsd16development:/home/user # cpucontrol -i 6 /dev/cpuctl0
cpuid level 0x6: 0x00000004 0x00000000 0x00000001 0x00000000
root@freebsd16development:/home/user # cpucontrol -m 0xe7 /dev/cpuctl0
MSR 0xe7: 0x00007fdf 0x22480f90
root@freebsd16development:/home/user # cpucontrol -m 0xe7 /dev/cpuctl0
MSR 0xe7: 0x00007fdf 0x2b8b48b0
root@freebsd16development:/home/user # cpucontrol -m 0xe8 /dev/cpuctl0
MSR 0xe8: 0x000050de 0x49270831
root@freebsd16development:/home/user # cpucontrol -m 0xe8 /dev/cpuctl0
MSR 0xe8: 0x000050de 0x4ad66ae6
root@freebsd16development:/home/user #

Based-on: QEMU v11.0.50 (commit 5611a9268d)

v1 -> v2 changelog:
  - Add migration flags

Anderson Nascimento (1):
  target/i386: Add support for KVM APERF/MPERF passthrough

 target/i386/cpu.c     | 18 +++++++++++++++++-
 target/i386/cpu.h     |  2 ++
 target/i386/kvm/kvm.c |  5 ++++-
 3 files changed, 23 insertions(+), 2 deletions(-)

-- 
2.54.0