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
