Date:	Wed, 30 Nov 2011 18:32:33 +0000
From:	Stefano Stabellini <stefano.stabellini@...citrix.com>
To:	Arnd Bergmann <arnd@...db.de>
CC:	Ian Campbell <Ian.Campbell@...rix.com>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	"linaro-dev@...ts.linaro.org" <linaro-dev@...ts.linaro.org>,
	Pawel Moll <pawel.moll@....com>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	Stefano Stabellini <Stefano.Stabellini@...citrix.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"virtualization@...ts.linux-foundation.org" 
	<virtualization@...ts.linux-foundation.org>,
	"android-virt@...ts.cs.columbia.edu" 
	<android-virt@...ts.cs.columbia.edu>,
	"embeddedxen-devel@...ts.sourceforge.net" 
	<embeddedxen-devel@...ts.sourceforge.net>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [Embeddedxen-devel] [Xen-devel] [ANNOUNCE] Xen port to Cortex-A15
 / ARMv7 with virt extensions

On Wed, 30 Nov 2011, Arnd Bergmann wrote:
> > In principal we could also offer the user options as to which particular
> > platform a guest looks like.
> 
> At least when using a qemu based simulation. Most platforms have some
> characteristics that are not meaningful in a classic virtualization
> scenario, but it would certainly be helpful to use the virtualization
> extensions to run a kernel that was built for a particular platform
> faster than with pure qemu, when you want to test that kernel image.
> 
> It has been suggested in the past that it would be nice to run the
> guest kernel built for the same platform as the host kernel by
> default, but I think it would be much better to have just one
> platform that we end up using for guests on any host platform,
> unless there is a strong reason to do otherwise.
> 
> There is also ongoing restructuring in the ARM Linux kernel to
> allow running the same kernel binary on multiple platforms. While
> there is still a lot of work to be done, you should assume that
> we will finish it before you see lots of users in production, there
> is no need to plan for the current one-kernel-per-board case.

It is very good to hear, I am counting on it.


> > > Ok. It would of course still be possible to agree on an argument passing
> > > convention so that we can share the macros used to issue the hcalls,
> > > even if the individual commands are all different.
> > 
> > I think it likely that we can all agree on a common calling convention
> > for N-argument hypercalls. It doubt there are that many useful choices
> > with conflicting requirements yet strongly compelling advantages.
> 
> Exactly. I think it's only lack of communication that has resulted in
> different interfaces for each hypervisor on the other architectures.

It is also due to history: on X86 it was possible to issue hypercalls to
Xen before VMCALL (the X86 version of HVC) was available.


> KVM and Xen at least both fall into the single-return-value category,
> so we should be able to agree on a calling conventions. KVM does not
> have an hcall API on ARM yet, and I see no reason not to use the
> same implementation that you have in the Xen guest.
> 
> Stefano, can you split out the generic parts of your asm/xen/hypercall.h
> file into a common asm/hypercall.h and submit it for review to the
> arm kernel list?

Sure, I can do that.
Usually the hypercall calling convention is very hypervisor specific,
but if it turns out that we have the same requirements I happy to design
a common interface.
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