Re: [PATCH v7 07/10] arm64: hyperv: Initialize hypervisor on boot

From: Arnd Bergmann <arnd@arndb.de>
To: Michael Kelley <mikelley@microsoft.com>
Cc: Will Deacon <will@kernel.org>, Ard Biesheuvel <ardb@kernel.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	"Mark.Rutland@arm.com" <Mark.Rutland@arm.com>,
	Marc Zyngier <maz@kernel.org>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>,
	gregkh <gregkh@linuxfoundation.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"linux-hyperv@vger.kernel.org" <linux-hyperv@vger.kernel.org>,
	linux-efi <linux-efi@vger.kernel.org>,
	linux-arch <linux-arch@vger.kernel.org>,
	"wei.liu@kernel.org" <wei.liu@kernel.org>,
	vkuznets <vkuznets@redhat.com>, KY Srinivasan <kys@microsoft.com>,
	Sunil Muthuswamy <sunilmut@microsoft.com>,
	Boqun Feng <boqun.feng@gmail.com>
Subject: Re: [PATCH v7 07/10] arm64: hyperv: Initialize hypervisor on boot
Date: Wed, 26 Aug 2020 09:18:53 +0200	[thread overview]
Message-ID: <CAK8P3a3fzXuyamf=sokW59=Ln-ULBBGaSHxKwUeqxT_4sNrQqw@mail.gmail.com> (raw)
In-Reply-To: <MW2PR2101MB1052AD42CC4F9A71F87EFE95D7570@MW2PR2101MB1052.namprd21.prod.outlook.com>

On Tue, Aug 25, 2020 at 11:20 PM Michael Kelley <mikelley@microsoft.com> wrote:
> From: Arnd Bergmann <arnd@arndb.de> Sent: Monday, August 24, 2020 11:34 AM
> > On Mon, Aug 24, 2020 at 6:48 PM Michael Kelley <mikelley@microsoft.com> wrote:
> >
> > I think this has come up before, and I still don't consider it an acceptable
> > hack to hook platform initialization code into the timer code.
> >
> > Please split out the timer into a standalone driver in drivers/clocksource
> > that can get reviewed by the clocksource maintainers.
>
> I see two related topics here.

Agreed

>  First, the Hyper-V clocksource driver is
> drivers/clocksource/hyperv_timer.c.  The code is architecture independent
> and is used today on the x86 side and for ARM64 in this patch series.  A few
> architecture specific calls are satisfied by code under arch/x86, and in this
> patch series, under arch/arm64.  Is there some aspect of this driver that
> needs reconsideration?  I just want to make sure to understand what you
> are getting at.

For the clocksource driver, I would like to see the arm64 specific bits
(the code you add in arch/arm64 that are only relevant to this driver)
moved out of arch/arm64 and into drivers/clocksource, in whatever
form the clocksource maintainers prefer. I would suggest having a
separate file that can get linked along with the architecture-independent
part of that driver.

> Second is the question of where/how to do Hyper-V specific initialization.
> I agree that hanging it off the timer initialization isn't a great approach.
> Should I add a Hyper-V specific initialization call at the appropriate point
> in the ARM64 init sequence?  The x86 side has some structure for handling
> multiple hypervisors, and the Hyper-V initialization code naturally plugs into
> that structure.  I'm certainly open to suggestions on the best way to handle
> it for ARM64.

Yes, that is where I was getting at. Maybe the x86 abstraction for handling
multiple hypervisors can be lifted out of arch/x86/ into common code?

       Arnd