Re: [PATCH] arm64: xen: Implement EFI reset_system callback

From: Matt Fleming <matt@codeblueprint.co.uk>
To: Daniel Kiper <daniel.kiper@oracle.com>
Cc: Mark Rutland <mark.rutland@arm.com>,
	Julien Grall <julien.grall@arm.com>,
	Juergen Gross <jgross@suse.com>,
	Boris Ostrovsky <boris.ostrovsky@oracle.com>,
	sstabellini@kernel.org, linux-kernel@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org, xen-devel@lists.xen.org,
	Ard Biesheuvel <ard.biesheuvel@linaro.org>,
	linux-efi@vger.kernel.org
Subject: Re: [PATCH] arm64: xen: Implement EFI reset_system callback
Date: Wed, 19 Apr 2017 20:37:38 +0100	[thread overview]
Message-ID: <20170419193738.GM24360@codeblueprint.co.uk> (raw)
In-Reply-To: <20170419192906.GO16658@olila.local.net-space.pl>

On Wed, 19 Apr, at 09:29:06PM, Daniel Kiper wrote:
> On Tue, Apr 18, 2017 at 02:46:50PM +0100, Matt Fleming wrote:
> > On Thu, 06 Apr, at 04:55:11PM, Mark Rutland wrote:
> > >
> > > Please, let's keep the Xen knowledge constrained to the Xen EFI wrapper,
> > > rather than spreading it further.
> > >
> > > IMO, given reset_system is a *mandatory* function, the Xen wrapper
> > > should provide an implementation.
> > >
> > > I don't see why you can't implement a wrapper that calls the usual Xen
> > > poweroff/reset functions.
> >
> > I realise I'm making a sweeping generalisation, but adding
> > EFI_PARAVIRT is almost always the wrong thing to do.
> 
> Why?

Because it makes paravirt a special case, and there's usually very
little reason to make it special in the EFI code. Special-casing means
more branches, more code paths, a bigger testing matrix and more
complex code. 

EFI_PARAVIRT does have its uses, like for those scenarios where we
don't have a table of function pointers that can be overidden for
paravirt.

But we do have such a table for ->reset_system().