linux-kernel - Re: [RFC PATCH v9 6/8] psci: Add hvc call service for ptp

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Message-ID: <22ba1283a7b82f018c1fdf85414e5bfe@kernel.org>
Date:   Mon, 13 Jan 2020 11:16:26 +0000
From:   Marc Zyngier <maz@...nel.org>
To:     Jianyong Wu <Jianyong.Wu@....com>
Cc:     netdev@...r.kernel.org, yangbo.lu@....com, john.stultz@...aro.org,
        tglx@...utronix.de, pbonzini@...hat.com,
        sean.j.christopherson@...el.com, richardcochran@...il.com,
        Mark Rutland <Mark.Rutland@....com>, will@...nel.org,
        Suzuki Poulose <Suzuki.Poulose@....com>,
        Steven Price <Steven.Price@....com>,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        kvmarm@...ts.cs.columbia.edu, kvm@...r.kernel.org,
        Steve Capper <Steve.Capper@....com>,
        Kaly Xin <Kaly.Xin@....com>, Justin He <Justin.He@....com>,
        nd <nd@....com>
Subject: Re: [RFC PATCH v9 6/8] psci: Add hvc call service for ptp_kvm.

Hi Jianyong,

On 2020-01-13 10:30, Jianyong Wu wrote:
> Hi Marc,
> 
>> -----Original Message-----
>> From: Marc Zyngier <maz@...nel.org>
>> Sent: Friday, January 10, 2020 6:56 PM
>> NV breaks that assumtion, because the guest hypervisor is using the 
>> physical
>> counter. Also, let's not forget that the hypercall isn't Linux 
>> specific.
>> I can write my own non-Linux guest and still use this hypercall. 
>> Nothing in
>> there says that I can't use the physical counter if I want to.
>> 
>> So somehow, you need to convey the the hypervisor the notion of 
>> *which*
>> counter the guest uses.
>> 
>> Does it make sense? Or am I missing something?
>> 
> I know what you say. Let me try to solve this problem.
> 	Step 0, summary out all the conditions we should process, which will
> sever as branch condition.(now only normal virt and nested virt, I
> think)

No. You shouldn't think of the various use cases, but of which time
references a guest can use. You don't need nested virt to use the 
physical
counter, for example.

> 	Step 1, figure out the set of reference counter value used by guest
> in all condition.

That should be for the guest to tell you when it calls into the PV 
service.

> 	Step 2, determine which reference counter value will be used by guest
> in a certain condition in hypercall.
> In step 1, can we give the set only 2 elements that one is physical
> counter the other is virtual counter?

I don't think returning the two values is useful. Just return what the
guest asks for.

> For step 2, I have no idea for that now. can you give me some hint 
> about it?

Just expand your SMC call to take a parameter indicating the reference
counter, and return the sampled (or computed) value corresponding to
that counter.

         M.
-- 
Jazz is not dead. It just smells funny...