Messages in this thread | | | Date | Wed, 22 Dec 2021 14:56:55 +0800 | From | Like Xu <> | Subject | Re: [PATCH] KVM: X86: Emulate APERF/MPERF to report actual VCPU frequency |
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On 24/6/2020 4:34 am, Jim Mattson wrote: > On Tue, Jun 23, 2020 at 12:05 PM Sean Christopherson > <sean.j.christopherson@intel.com> wrote: >> >> On Tue, Jun 23, 2020 at 11:39:16AM -0700, Jim Mattson wrote: >>> On Tue, Jun 23, 2020 at 11:29 AM Sean Christopherson >>> <sean.j.christopherson@intel.com> wrote: >>>> >>>> On Tue, Jun 23, 2020 at 02:35:30PM +0800, Like Xu wrote: >>>>> The aperf/mperf are used to report current CPU frequency after 7d5905dc14a >>>>> "x86 / CPU: Always show current CPU frequency in /proc/cpuinfo". But guest >>>>> kernel always reports a fixed VCPU frequency in the /proc/cpuinfo, which >>>>> may confuse users especially when turbo is enabled on the host. >>>>> >>>>> Emulate guest APERF/MPERF capability based their values on the host. >>>>> >>>>> Co-developed-by: Li RongQing <lirongqing@baidu.com> >>>>> Signed-off-by: Li RongQing <lirongqing@baidu.com> >>>>> Reviewed-by: Chai Wen <chaiwen@baidu.com> >>>>> Reviewed-by: Jia Lina <jialina01@baidu.com> >>>>> Signed-off-by: Like Xu <like.xu@linux.intel.com> >>>>> --- >>>> >>>> ... >>>> >>>>> @@ -8312,7 +8376,7 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu) >>>>> dm_request_for_irq_injection(vcpu) && >>>>> kvm_cpu_accept_dm_intr(vcpu); >>>>> fastpath_t exit_fastpath; >>>>> - >>>>> + u64 enter_mperf = 0, enter_aperf = 0, exit_mperf = 0, exit_aperf = 0; >>>>> bool req_immediate_exit = false; >>>>> >>>>> if (kvm_request_pending(vcpu)) { >>>>> @@ -8516,8 +8580,17 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu) >>>>> vcpu->arch.switch_db_regs &= ~KVM_DEBUGREG_RELOAD; >>>>> } >>>>> >>>>> + if (unlikely(vcpu->arch.hwp.hw_coord_fb_cap)) >>>>> + get_host_amperf(&enter_mperf, &enter_aperf); >>>>> + >>>>> exit_fastpath = kvm_x86_ops.run(vcpu); >>>>> >>>>> + if (unlikely(vcpu->arch.hwp.hw_coord_fb_cap)) { >>>>> + get_host_amperf(&exit_mperf, &exit_aperf); >>>>> + vcpu_update_amperf(vcpu, get_amperf_delta(enter_aperf, exit_aperf), >>>>> + get_amperf_delta(enter_mperf, exit_mperf)); >>>>> + } >>>>> + >>>> >>>> Is there an alternative approach that doesn't require 4 RDMSRs on every VMX >>>> round trip? That's literally more expensive than VM-Enter + VM-Exit >>>> combined.
It looks like we have quite a few users who are expecting this feature in different scenarios.
I will add a fast path for RO usage and a slow path if the guest tries to change the AMPERF values.
>>>> >>>> E.g. what about adding KVM_X86_DISABLE_EXITS_APERF_MPERF and exposing the >>>> MSRs for read when that capability is enabled? >>> >>> When would you load the hardware MSRs with the guest/host values? >> >> Ugh, I was thinking the MSRs were read-only. > > EVen if they were read-only, they should power on to zero, and they > will most likely not be zero when a guest powers on.
Can we assume that "not zero when the guest is on" will not harm any guests ?
> >> Doesn't this also interact with TSC scaling? > > Yes, it should!
We have too much of a historical burden on TSC emulations.
For practical reasons, what if we only expose the AMPERF cap if the host/guest has both CONSTANT_TSC and NONSTOP_TSC ?
One more design concern, I wonder if it is *safe* for the guest to read amperf on pCPU[x] the first time and on pCPU[y] the next time.
Any input ?
Thanks, Like Xu
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