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(-)
