RE: [PATCH v15 7/9] ptp: arm/arm64: Enable ptp_kvm for arm/arm64

From: Jianyong Wu <Jianyong.Wu@arm.com>
To: Marc Zyngier <maz@kernel.org>
Cc: "netdev@vger.kernel.org" <netdev@vger.kernel.org>,
	"yangbo.lu@nxp.com" <yangbo.lu@nxp.com>,
	"john.stultz@linaro.org" <john.stultz@linaro.org>,
	"tglx@linutronix.de" <tglx@linutronix.de>,
	"pbonzini@redhat.com" <pbonzini@redhat.com>,
	"sean.j.christopherson@intel.com"
	<sean.j.christopherson@intel.com>,
	"richardcochran@gmail.com" <richardcochran@gmail.com>,
	Mark Rutland <Mark.Rutland@arm.com>,
	"will@kernel.org" <will@kernel.org>,
	Suzuki Poulose <Suzuki.Poulose@arm.com>,
	Andre Przywara <Andre.Przywara@arm.com>,
	Steven Price <Steven.Price@arm.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"linux-arm-kernel@lists.infradead.org" 
	<linux-arm-kernel@lists.infradead.org>,
	"kvmarm@lists.cs.columbia.edu" <kvmarm@lists.cs.columbia.edu>,
	"kvm@vger.kernel.org" <kvm@vger.kernel.org>,
	Steve Capper <Steve.Capper@arm.com>,
	Justin He <Justin.He@arm.com>, nd <nd@arm.com>
Subject: RE: [PATCH v15 7/9] ptp: arm/arm64: Enable ptp_kvm for arm/arm64
Date: Tue, 24 Nov 2020 10:18:41 +0000	[thread overview]
Message-ID: <HE1PR0802MB25557885DA7391D52353B7C5F4FB0@HE1PR0802MB2555.eurprd08.prod.outlook.com> (raw)
In-Reply-To: <5dc5480d125ace6566ae616206c3ce3f@kernel.org>

Hi Marc,

> -----Original Message-----
> From: Marc Zyngier <maz@kernel.org>
> Sent: Tuesday, November 24, 2020 5:05 PM
> To: Jianyong Wu <Jianyong.Wu@arm.com>
> Cc: netdev@vger.kernel.org; yangbo.lu@nxp.com; john.stultz@linaro.org;
> tglx@linutronix.de; pbonzini@redhat.com; sean.j.christopherson@intel.com;
> richardcochran@gmail.com; Mark Rutland <Mark.Rutland@arm.com>;
> will@kernel.org; Suzuki Poulose <Suzuki.Poulose@arm.com>; Andre
> Przywara <Andre.Przywara@arm.com>; Steven Price
> <Steven.Price@arm.com>; linux-kernel@vger.kernel.org; linux-arm-
> kernel@lists.infradead.org; kvmarm@lists.cs.columbia.edu;
> kvm@vger.kernel.org; Steve Capper <Steve.Capper@arm.com>; Justin He
> <Justin.He@arm.com>; nd <nd@arm.com>
> Subject: Re: [PATCH v15 7/9] ptp: arm/arm64: Enable ptp_kvm for
> arm/arm64
> 
> Jianyong,
> 
> On 2020-11-24 05:37, Jianyong Wu wrote:
> > Hi Marc,
> 
> [...]
> 
> >> > +
> >> 	arm_smccc_1_1_invoke(ARM_SMCCC_VENDOR_HYP_KVM_PTP_FU
> >> NC_ID,
> >> > +			     ARM_PTP_NONE_COUNTER, &hvc_res);
> >>
> >> I really don't see the need to use a non-architectural counter ID.
> >> Using the virtual counter ID should just be fine, and shouldn't lead
> >> to any issue.
> >>
> >
> >> Am I missing something?
> >
> > In this function, no counter data is needed. If virtual counter ID is
> > used here, user may be confused with why we get virtual counter
> > data and do nothing with it. So I explicitly add a new "NONE" counter
> > ID to make it clear.
> >
> > WDYT?
> 
> ITIABI (I Think It's A Bad Idea). There are two counters, and the API
> allows to retrieve the data for any of these two. If the "user" doesn't
> want to do anything with the data, that's their problem.
> 
> Here, you can just sue the virtual counter, and that will give you the
> exact same semantic, without inventing non-architectural state.
> 
OK, that's it.

Thanks
Jianyong Wu

> Thanks,
> 
>          M.
> --
> Jazz is not dead. It just smells funny...