On 7/4/2023 10:56 pm, Sean Christopherson wrote:
On Fri, Apr 07, 2023, Like Xu wrote:
On 7/4/2023 10:18 am, Sean Christopherson wrote:
Wait, really? VMRUN is counted if and only if it enters to a CPL0 guest? Can
someone from AMD confirm this? I was going to say we should just treat this as
"normal" behavior, but counting CPL0 but not CPL>0 is definitely quirky.
VMRUN is only counted on a CPL0-target (branch) instruction counter.
Yes or no question: if KVM does VMRUN and a PMC is programmed to count _all_ taken
branches, will the PMC count VMRUN as a branch if guest CPL>0 according to the VMCB?
YES, my quick tests (based on run_in_user() from KUT on Zen4) show:
EVENTSEL_GUESTONLY + EVENTSEL_ALL + VMRUN_to_USR -> AMD_ZEN_BR_RETIRED + 1
EVENTSEL_GUESTONLY + EVENTSEL_ALL + VMRUN_to_OS -> AMD_ZEN_BR_RETIRED + 1
EVENTSEL_GUESTONLY + EVENTSEL_USR + VMRUN_to_USR -> AMD_ZEN_BR_RETIRED + 1
EVENTSEL_GUESTONLY + EVENTSEL_OS + VMRUN_to_OS -> AMD_ZEN_BR_RETIRED + 1
VENTSEL_GUESTONLY + EVENTSEL_OS + VMRUN_to_USR -> No change
VENTSEL_GUESTONLY + EVENTSEL_USR + VMRUN_to_OS -> No change
I'm actually not surprised and related test would be posted later.
This issue makes a guest CPL0-target instruction counter inexplicably
increase, as if it would have been under-counted before the virtualization
instructions were counted.
Heh, it's very much explicable, it's just not desirable, and you and I would argue
that it's also incorrect.
This is completely inaccurate from the end guest pmu user's perspective.
I have a toy that looks like virtio-pmu, through which guest users can get
hypervisor performance data.
But the side effect of letting the guest see the VMRUN instruction by default is
unacceptable, isn't it ?
AMD folks, are there plans to document this as an erratum? I agree with Like that
counting VMRUN as a taken branch in guest context is a CPU bug, even if the behavior
is known/expected.
+CC: Santosh, Tom, Ananth
