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

[Arnd Bergmann]

On Wed, Aug 26, 2020 at 12:04 AM Michael Kelley <mikelley@xxxxxxxxxxxxx> wrote:
> From: Arnd Bergmann <arnd@xxxxxxxx> 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_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.

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.

       Arnd