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From: Arnd Bergmann <>
To: Michael Kelley <>
Cc: Will Deacon <>, Ard Biesheuvel <>,
	Catalin Marinas <>,
	"" <>,
	Marc Zyngier <>,
	Linux ARM <>,
	gregkh <>,
	"" <>,
	"" <>,
	linux-efi <>,
	linux-arch <>,
	"" <>,
	vkuznets <>, KY Srinivasan <>,
	Sunil Muthuswamy <>,
	Boqun Feng <>
Subject: Re: [PATCH v7 05/10] arm64: hyperv: Add interrupt handlers for VMbus and stimer
Date: Wed, 26 Aug 2020 09:14:39 +0200
Message-ID: <> (raw)
In-Reply-To: <>

On Wed, Aug 26, 2020 at 12:04 AM Michael Kelley <> wrote:
> From: Arnd Bergmann <> Sent: Monday, August 24, 2020 11:54 AM

> >
> > I'm not sure what the correct solution should be, but what I'd try to
> > do here is to move every function that just considers the platform
> > rather than the architecture somewhere into drivers/hv where it
> > can be linked into the same modules as the existing files when
> > building for arm64, while trying to keep architecture specific code
> > in the header file where it can be included from those modules.
> OK.  The concept of separating platform from architecture makes
> sense to me.  The original separation of the Hyper-V code into
> architecture independent portions and x86-specific portions could
> use some tweaking now that we're dealing with n=2 architectures.  With
> that tweaking, I can reduce the amount of Hyper-V code under arch/x86
> and under arch/arm64.
> On the flip side, the Hyper-V implementation on x86 and ARM64 has
> differences that are semi-related to the architecture.  For example, on
> x86 Hyper-V uses synthetic MSRs for a lot of guest-hypervisor setup, while
> hypercalls are required on ARM64.  So I'm assuming those differences
> will end up in code under arch/x86 and arch/arm64.

Yes, that absolutely makes sense.

> Arguably, I could introduce a level of indirection (such as
> CONFIG_HYPERV_USE_HYPERCALLS) to distinguish the two behaviors.
> The selection would be tied to the architecture, and then code in
> drivers/hv can #ifdef the two cases.  But I wonder if getting code out of
> arch/x86 and arch/arm64 is worth that additional messiness.

No, I think that would take it a little too far, and conflicts with the
generic rule that code under drivers/* should be written to be portable
even if can only run on a particular target platform.

> Looking at the Xen code in drivers/xen, it looks like a lot of the Xen functionality
> is implemented in hypercalls that can be consistent across architectures,
> though I was a bit surprised to see a dozen or so instances of #ifdef CONFIG_X86.
> Xen also #ifdefs on PV vs. PVHVM, which may handle some architecture
> differences implicitly.  But I'm assuming that doing #ifdef <architecture>
> in the Hyper-V code in order to reduce code under arch/x86 or arch/arm64
> is not the right way to go.

In general that is true, adding a lot of #ifdefs makes code less readable and
harder to test. OTOH there are cases where a single #ifdef can be useful when
it avoids adding a larger amount of complexity elsewhere. Many subsystems
try to restrict the #ifdef checks to header files while keeping the
drivers/* code
free of them.


  reply index

Thread overview: 25+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-08-24 16:46 [PATCH v7 00/10] Enable Linux guests on Hyper-V on ARM64 Michael Kelley
2020-08-24 16:46 ` [PATCH v7 01/10] arm/arm64: smccc-1.1: Add vendor specific owner definition Michael Kelley
2020-08-24 16:46 ` [PATCH v7 02/10] arm64: hyperv: Add core Hyper-V include files Michael Kelley
2020-08-24 18:38   ` Arnd Bergmann
2020-08-24 16:46 ` [PATCH v7 03/10] arm64: hyperv: Add hypercall and register access functions Michael Kelley
2020-08-24 16:46 ` [PATCH v7 04/10] arm64: hyperv: Add memory alloc/free functions for Hyper-V size pages Michael Kelley
2020-08-24 16:46 ` [PATCH v7 05/10] arm64: hyperv: Add interrupt handlers for VMbus and stimer Michael Kelley
2020-08-24 18:54   ` Arnd Bergmann
2020-08-25 22:04     ` Michael Kelley
2020-08-26  7:14       ` Arnd Bergmann [this message]
2020-08-24 16:46 ` [PATCH v7 06/10] arm64: hyperv: Add kexec and panic handlers Michael Kelley
2020-08-24 16:46 ` [PATCH v7 07/10] arm64: hyperv: Initialize hypervisor on boot Michael Kelley
2020-08-24 18:33   ` Arnd Bergmann
2020-08-25 21:20     ` Michael Kelley
2020-08-26  7:18       ` Arnd Bergmann
2020-08-24 16:46 ` [PATCH v7 08/10] Drivers: hv: vmbus: Add hooks for per-CPU IRQ Michael Kelley
2020-08-24 16:46 ` [PATCH v7 09/10] arm64: efi: Export screen_info Michael Kelley
2020-08-24 17:21   ` Ard Biesheuvel
2020-08-24 17:35   ` Greg KH
2020-08-24 17:40     ` Michael Kelley
2020-08-24 17:52       ` Greg KH
2020-08-24 16:46 ` [PATCH v7 10/10] Drivers: hv: Enable Hyper-V code to be built on ARM64 Michael Kelley
2020-08-24 17:24   ` Ard Biesheuvel
2020-08-24 17:28     ` Michael Kelley
2020-08-25  8:47     ` Ben Dooks

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