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

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Message-ID: <1bfccb79-5ace-0dba-a201-e069f77f740a@amd.com>
Date:   Wed, 28 Aug 2019 18:39:56 +0000
From:   "Suthikulpanit, Suravee" <Suravee.Suthikulpanit@....com>
To:     Alexander Graf <graf@...zon.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>
CC:     "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "rkrcmar@...hat.com" <rkrcmar@...hat.com>,
        "joro@...tes.org" <joro@...tes.org>,
        "jschoenh@...zon.de" <jschoenh@...zon.de>,
        "karahmed@...zon.de" <karahmed@...zon.de>,
        "rimasluk@...zon.com" <rimasluk@...zon.com>,
        "Grimm, Jon" <Jon.Grimm@....com>
Subject: Re: [PATCH v2 04/15] kvm: x86: Add per-VM APICv state debugfs

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).

> 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,
Suravee