On 6/2/26 04:20, Anderson Nascimento wrote:
> 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.
Mostly looks good, but please mark it as unmigratable as well (similar
to INVTSC which, not coincidentially, you're also using below).
Paolo
>
> 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="+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 #
>
> I am looking for feedback on the approach, specifically if tying
> this directly into `cpu-pm` makes the most sense to the
> maintainers.
>
> Based-on: QEMU v11.0.50 (commit 5611a9268d)
>
> Anderson Nascimento (1):
> target/i386: Add support for KVM APERF/MPERF passthrough
>
> target/i386/cpu.c | 17 ++++++++++++++++-
> target/i386/cpu.h | 2 ++
> target/i386/kvm/kvm.c | 5 ++++-
> 3 files changed, 22 insertions(+), 2 deletions(-)
>