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Message-ID: <9e898dae-c8a9-7b04-6165-ca4d7ddce988@gmail.com>
Date: Wed, 22 Dec 2021 14:56:55 +0800
From: Like Xu <like.xu.linux@...il.com>
To: Jim Mattson <jmattson@...gle.com>,
Sean Christopherson <sean.j.christopherson@...el.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>,
kvm list <kvm@...r.kernel.org>,
Vitaly Kuznetsov <vkuznets@...hat.com>,
Wanpeng Li <wanpengli@...cent.com>,
Joerg Roedel <joro@...tes.org>, wei.huang2@....com,
Peter Zijlstra <peterz@...radead.org>,
Thomas Gleixner <tglx@...utronix.de>,
LKML <linux-kernel@...r.kernel.org>,
Li RongQing <lirongqing@...du.com>,
Like Xu <likexu@...cent.com>
Subject: Re: [PATCH] KVM: X86: Emulate APERF/MPERF to report actual VCPU
frequency
On 24/6/2020 4:34 am, Jim Mattson wrote:
> On Tue, Jun 23, 2020 at 12:05 PM Sean Christopherson
> <sean.j.christopherson@...el.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@...el.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@...du.com>
>>>>> Signed-off-by: Li RongQing <lirongqing@...du.com>
>>>>> Reviewed-by: Chai Wen <chaiwen@...du.com>
>>>>> Reviewed-by: Jia Lina <jialina01@...du.com>
>>>>> Signed-off-by: Like Xu <like.xu@...ux.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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