linux-kernel - Re: [PATCH v2 04/15] kvm: x86: Add per-VM APICv state debugfs

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Message-ID: <9A47CE02-18B0-40C3-962A-D317A32F2073@amazon.de>
Date:   Wed, 28 Aug 2019 19:36:03 +0000
From:   "Graf (AWS), Alexander" <graf@...zon.de>
To:     "Suthikulpanit, Suravee" <Suravee.Suthikulpanit@....com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "rkrcmar@...hat.com" <rkrcmar@...hat.com>,
        "joro@...tes.org" <joro@...tes.org>,
        "Schoenherr, Jan H." <jschoenh@...zon.de>,
        "Raslan, KarimAllah" <karahmed@...zon.de>,
        "Lukaszewicz, Rimas" <rimasluk@...zon.com>,
        "Grimm, Jon" <Jon.Grimm@....com>
Subject: Re: [PATCH v2 04/15] kvm: x86: Add per-VM APICv state debugfs



> Am 28.08.2019 um 20:41 schrieb Suthikulpanit, Suravee <Suravee.Suthikulpanit@....com>:
> 
> Alex,
> 
>> On 8/27/19 3:20 AM, Alexander Graf wrote:
>> 
>> 
>>> On 26.08.19 21:41, Suthikulpanit, Suravee wrote:
>>> Alex,
>>> 
>>>> On 8/19/2019 4:57 AM, Alexander Graf wrote:
>>>> 
>>>> 
>>>>> On 15.08.19 18:25, Suthikulpanit, Suravee wrote:
>>>>> Currently, there is no way to tell whether APICv is active
>>>>> on a particular VM. This often cause confusion since APICv
>>>>> can be deactivated at runtime.
>>>>> 
>>>>> Introduce a debugfs entry to report APICv state of a VM.
>>>>> This creates a read-only file:
>>>>> 
>>>>>      /sys/kernel/debug/kvm/70860-14/apicv-state
>>>>> 
>>>>> Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@....com>
>>>> 
>>>> Shouldn't this first and foremost be a VM ioctl so that user space 
>>>> can inquire its own state?
>>>> 
>>>> 
>>>> Alex
>>> 
>>> I introduce this mainly for debugging similar to how KVM is currently 
>>> provides
>>> some per-VCPU information:
>>> 
>>>       /sys/kernel/debug/kvm/15957-14/vcpu0/
>>>           lapic_timer_advance_ns
>>>           tsc-offset
>>>           tsc-scaling-ratio
>>>           tsc-scaling-ratio-frac-bits
>>> 
>>> I'm not sure if this needs to be VM ioctl at this point. If this 
>>> information is
>>> useful for user-space tool to inquire via ioctl, we can also provide it.
>> 
>> I'm mostly thinking of something like "info apic" in QEMU which to me 
>> seems like the natural place for APIC information exposure to a user.
> 
> I could not find QEMU "info apic". I assume you meant "info lapic", 
> which provides information specific to each local APIC (i.e. per-vcpu).

Or maybe even "info kvm" :). I think you get the point.

> 
>> The problem with debugfs is that it's not accessible to the user that 
>> created the VM, but only root, right?
> 
> Hm, you are right. I'll also look into also adding ioctl interface then.

Thanks!

Alex




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