On Mon, Feb 27, 2012 at 07:48:45PM +0000, Ian Campbell wrote: > On Mon, 2012-02-27 at 17:53 +0000, Dave Martin wrote: > > On Thu, Feb 23, 2012 at 05:48:22PM +0000, Stefano Stabellini wrote: > > > We need a register to pass the hypercall number because we might not > > > know it at compile time and HVC only takes an immediate argument. > > > > > > Among the available registers r12 seems to be the best choice because it > > > is defined as "intra-procedure call scratch register". > > > > This would be massively simplified if you didn't try to inline the HVC. > > Does it really need to be inline? > > > > > +#define __HYPERCALL ".word 0xe1400070 + " __HVC_IMM(XEN_HYPERCALL_TAG) > > > > Please, do not do this. It won't work in Thumb, where the encodings are > > different. > > > > It is reasonable to expect anyone building Xen to have reasonably new > > tools, you you can justifiably use > > > > AFLAGS_thisfile.o := -Wa,-march=armv7-a+virt > > > > in the Makefile and just use the hvc instruction directly. > > Our aim is for guest kernel binaries not to be specific to Xen -- i.e. > they should be able to run on baremetal and other hypervisors as well. > The differences should only be in the device-tree passed to the kernel. > > > Of course, this is only practical if the HVC invocation is not inlined. > > I suppose we could make the stub functions out of line, we just copied > what Xen does on x86. > > The only thing which springs to mind is that 5 argument hypercalls will > end up pushing the fifth argument to the stack only to pop it back into > r4 for the hypercall and IIRC it also needs to preserve r4 (callee saved > reg) which is going to involve some small amount of code to move stuff > around too. > > So by inlining the functions we avoid some thunking because the compiler > would know exactly what was happening at the hypercall site. True ... > > We don't currently have any 6 argument hypercalls but the same would > extend there. > > > If we can't avoid macro-ising HVC, we should do it globally, not locally > > to the Xen code. That way we at least keep all the horror in one place. > > That sounds like a good idea to me. > > Given that Stefano is proposing to make the ISS a (per-hypervisor) > constant we could consider just defining the Thumb and non-Thumb > constants instead of doing all the construction with the __HVC_IMM stuff > -- that would remove a big bit of the macroization. It's not quite as simple as that -- emitting instructions using data directives is not endianness safe, and even in the cases where .long gives the right result for ARM, it gives the wrong result for 32-bit Thumb instructions if the opcode is given in human-readable order. I was trying to solve the same problem for the kvm guys with some global macros -- I'm aiming to get a patch posted soon, so I'll make sure you're on CC. Cheers ---Dave -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
