linux-kernel - Re: [PATCH v7 12/17] ARM64: ACPI: Check if it runs on Xen to enable or disable ACPI

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Message-ID: <20160329163147.GB27223@leverpostej>
Date:	Tue, 29 Mar 2016 17:31:47 +0100
From:	Mark Rutland <mark.rutland@....com>
To:	Will Deacon <will.deacon@....com>
Cc:	Shannon Zhao <shannon.zhao@...aro.org>,
	linux-arm-kernel@...ts.infradead.org,
	stefano.stabellini@...rix.com, david.vrabel@...rix.com,
	devicetree@...r.kernel.org, linux-efi@...r.kernel.org,
	linux-kernel@...r.kernel.org, catalin.marinas@....com,
	julien.grall@....com, peter.huangpeng@...wei.com,
	xen-devel@...ts.xen.org, zhaoshenglong@...wei.com
Subject: Re: [PATCH v7 12/17] ARM64: ACPI: Check if it runs on Xen to enable
 or disable ACPI

On Tue, Mar 29, 2016 at 05:18:38PM +0100, Will Deacon wrote:
> On Thu, Mar 24, 2016 at 10:44:31PM +0800, Shannon Zhao wrote:
> > When it's a Xen domain0 booting with ACPI, it will supply a /chosen and
> > a /hypervisor node in DT. So check if it needs to enable ACPI.
> > 
> > Signed-off-by: Shannon Zhao <shannon.zhao@...aro.org>
> > Reviewed-by: Stefano Stabellini <stefano.stabellini@...citrix.com>
> > Acked-by: Hanjun Guo <hanjun.guo@...aro.org>
> > ---
> >  arch/arm64/kernel/acpi.c | 12 ++++++++----
> >  1 file changed, 8 insertions(+), 4 deletions(-)
> > 
> > diff --git a/arch/arm64/kernel/acpi.c b/arch/arm64/kernel/acpi.c
> > index d1ce8e2..4e92be0 100644
> > --- a/arch/arm64/kernel/acpi.c
> > +++ b/arch/arm64/kernel/acpi.c
> > @@ -67,10 +67,13 @@ static int __init dt_scan_depth1_nodes(unsigned long node,
> >  {
> >  	/*
> >  	 * Return 1 as soon as we encounter a node at depth 1 that is
> > -	 * not the /chosen node.
> > +	 * not the /chosen node, or /hypervisor node when running on Xen.
> >  	 */
> > -	if (depth == 1 && (strcmp(uname, "chosen") != 0))
> > -		return 1;
> > +	if (depth == 1 && (strcmp(uname, "chosen") != 0)) {
> > +		if (!xen_initial_domain() || (strcmp(uname, "hypervisor") != 0))
> > +			return 1;
> > +	}
> 
> Hmm, but xen_initial_domain() is false when xen isn't being used at all,
> so it feels to me like this is a bit too far-reaching and is basically
> claiming the "/hypervisor" namespace for Xen. Couldn't it be renamed to
> "xen,hypervisor" or something?
> 
> Mark, got any thoughts on this?

The node has a compatible string, "xen,xen" per [1], which would tell us
absolutely that xen is present. I'd be happy checking for that
explicitly.

In patch 11 fdt_find_hyper_node checks the compatible string. We could
factor that out into a helper like is_xen_node(node) and use it here
too.

If in future we want to do the same for other hypervisors we can add
explicit checks for their compatible strings and/or wrap that in a more
generic helper to cater for any hypervisor-specific details.

Thanks,
Mark.

[1] Documentation/devicetree/bindings/arm/xen.txt