From mboxrd@z Thu Jan 1 00:00:00 1970 From: Dave Martin Subject: Re: [PATCH-WIP 01/13] xen/arm: use r12 to pass the hypercall number to the hypervisor Date: Tue, 28 Feb 2012 09:46:16 +0000 Message-ID: <20120228094616.GA2063@linaro.org> References: <1330019314-20865-1-git-send-email-stefano.stabellini@eu.citrix.com> <20120227175327.GA2023@linaro.org> <1330372125.10008.47.camel@dagon.hellion.org.uk> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Cc: "xen-devel-GuqFBffKawuULHF6PoxzQEEOCMrvLtNR@public.gmane.org" , "linaro-dev-cunTk1MwBs8s++Sfvej+rw@public.gmane.org" , "kvm-u79uwXL29TY76Z2rM5mHXA@public.gmane.org" , "arnd-r2nGTMty4D4@public.gmane.org" , "catalin.marinas-5wv7dgnIgG8@public.gmane.org" , "linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org" , David Vrabel , "linux-arm-kernel-IAPFreCvJWM7uuMidbF8XUB+6BGkLq7r@public.gmane.org" To: Ian Campbell Return-path: Content-Disposition: inline In-Reply-To: <1330372125.10008.47.camel-ztPmHsLffjjnO4AKDKe2m+kiAK3p4hvP@public.gmane.org> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: linaro-dev-bounces-cunTk1MwBs8s++Sfvej+rw@public.gmane.org Errors-To: linaro-dev-bounces-cunTk1MwBs8s++Sfvej+rw@public.gmane.org List-Id: kvm.vger.kernel.org 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