RE: [PATCH v7 05/10] arm64: hyperv: Add interrupt handlers for VMbus and stimer

[Michael Kelley]

From: Arnd Bergmann <arnd@xxxxxxxx> Sent: Monday, August 24, 2020 11:54 AM
> 
> On Mon, Aug 24, 2020 at 6:46 PM Michael Kelley <mikelley@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx>
> > ---
> >  arch/arm64/hyperv/Makefile        |   2 +-
> >  arch/arm64/hyperv/mshyperv.c      | 133
> ++++++++++++++++++++++++++++++++++++++
> >  arch/arm64/include/asm/mshyperv.h |  70 ++++++++++++++++++++
> 
> I still have the feeling that most of the code in arch/arm64/hyperv/ is
> misplaced: the only callers are loadable modules in drivers/hv/, and the
> code is not really part of the architecture but part of the platform.
> 
> For the arm64 architecture, we have a rule that platform specific
> code belongs into device drivers rather than into the architecture
> code as we used to do in the linux-2.6 days for arch/arm/.
> 
> I don't see hyperv being virtual rather than an SoC as a differentiator
> either; it's still just one of many platforms. If you look at
> arch/arm64/xen/, you can see that they have managed to get
> to a much simpler implementation in comparison.
> 
> 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.  Arguably, I could
introduce a level of indirection (such as CONFIG_HYPERV_USE_MSRS vs.
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.

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.

Michael

> 
>       Arnd