linux-kernel - Re: [Android-virt] [Embeddedxen-devel] [Xen-devel] [ANNOUNCE] Xen port to Cortex-A15 / ARMv7 with virt extensions

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Message-ID: <20111201151043.GG27394@arm.com>
Date:	Thu, 1 Dec 2011 15:10:43 +0000
From:	Catalin Marinas <catalin.marinas@....com>
To:	Ian Campbell <Ian.Campbell@...rix.com>
Cc:	Stefano Stabellini <Stefano.Stabellini@...citrix.com>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	"linaro-dev@...ts.linaro.org" <linaro-dev@...ts.linaro.org>,
	Arnd Bergmann <arnd@...db.de>, Pawel Moll <Pawel.Moll@....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>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"embeddedxen-devel@...ts.sourceforge.net" 
	<embeddedxen-devel@...ts.sourceforge.net>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [Android-virt] [Embeddedxen-devel] [Xen-devel] [ANNOUNCE] Xen
 port to Cortex-A15 / ARMv7 with virt extensions

On Thu, Dec 01, 2011 at 10:26:37AM +0000, Ian Campbell wrote:
> On Wed, 2011-11-30 at 18:32 +0000, Stefano Stabellini wrote:
> > On Wed, 30 Nov 2011, Arnd Bergmann wrote:
> > > 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.
> 
> I expect the only real decision to be made is hypercall page vs. raw hvc
> instruction.
> 
> The page was useful on x86 where there is a variety of instructions
> which could be used (at least for PV there was systenter/syscall/int, I
> think vmcall instruction differs between AMD and Intel also) and gives
> some additional flexibility. It's hard to predict but I don't think I'd
> expect that to be necessary on ARM.
> 
> Another reason for having a hypercall page instead of a raw instruction
> might be wanting to support 32 bit guests (from ~today) on a 64 bit
> hypervisor in the future and perhaps needing to do some shimming/arg
> translation. It would be better to aim for having the interface just be
> 32/64 agnostic but mistakes do happen.

Given the way register banking is done on AArch64, issuing an HVC on a
32-bit guest OS doesn't require translation on a 64-bit hypervisor. We
have a similar implementation at the SVC level (for 32-bit user apps on
a 64-bit kernel), the only modification was where a 32-bit SVC takes a
64-bit parameter in two separate 32-bit registers, so packing needs to
be done in a syscall wrapper.

I'm not closely involved with any of the Xen or KVM work but I would
vote for using HVC than a hypercall page.

-- 
Catalin
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