Re: [PATCH v6 05/10] arm64: hyperv: Add interrupt handlers for VMbus and stimer

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 <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>,
	"olaf@aepfle.de" <olaf@aepfle.de>,
	Andy Whitcroft <apw@canonical.com>,
	vkuznets <vkuznets@redhat.com>, Jason Wang <jasowang@redhat.com>,
	"marcelo.cerri@canonical.com" <marcelo.cerri@canonical.com>,
	KY Srinivasan <kys@microsoft.com>,
	Sunil Muthuswamy <sunilmut@microsoft.com>,
	Boqun Feng <boqun.feng@gmail.com>
Subject: Re: [PATCH v6 05/10] arm64: hyperv: Add interrupt handlers for VMbus and stimer
Date: Wed, 18 Mar 2020 10:52:30 +0100	[thread overview]
Message-ID: <CAK8P3a1zm2UH-S6eN4qMO_o1M8b0ieMq+2tQGVKmtC-UKeZUTQ@mail.gmail.com> (raw)
In-Reply-To: <MW2PR2101MB1052B9C24DAB19FBBD818347D7F70@MW2PR2101MB1052.namprd21.prod.outlook.com>

On Wed, Mar 18, 2020 at 1:16 AM Michael Kelley <mikelley@microsoft.com> wrote:
> From: Arnd Bergmann <arnd@arndb.de>
> > On Sat, Mar 14, 2020 at 4:36 PM Michael Kelley <mikelley@microsoft.com> wrote:
> > >
> > > Add ARM64-specific code to set up and handle the interrupts
> > > generated by Hyper-V for VMbus messages and for stimer expiration.
> > >
> > > This code is architecture dependent and is mostly driven by
> > > architecture independent code in the VMbus driver and the
> > > Hyper-V timer clocksource driver.
> > >
> > > This code is built only when CONFIG_HYPERV is enabled.
> > >
> > > Signed-off-by: Michael Kelley <mikelley@microsoft.com>
> >
> > This looks like it should be a nested irqchip driver instead, so your
> > device drivers can use the normal request_irq() functions etc.
> >
> > Is anything preventing you from doing that? If so, please describe
> > that in the changelog and in a comment in the driver.
> >
>
> As mentioned in my reply on Patch 1, Hyper-V offers a limited synthetic
> interrupt controller managed by Linux code that's been around the last
> 10 years on the x86 side.   For reasons that pre-date me, it was not written
> as an irqchip driver.

I think the reason is just that 10 years ago, we did not have the concept
of irqchips as device drivers.

> Modulo the small routines you see in this patch, the code is architecture
> independent, and it seems we ought to keep the high degree of commonality.
> Re-architecting the arch independent code to model as an irqchip driver seems
> to carry some risk to the x86 side that has a lot of real-world usage today, but
> I'll take a look and see what the risks look like and if it adds any clarity.

How many drivers link against the custom interface? If it's less than 10,
making it a real driver is probably not too hard to do.

       Arnd