linux-kernel - Re: [PATCH] arm64: xen: Implement EFI reset

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Message-ID: <20170419193738.GM24360@codeblueprint.co.uk>
Date:   Wed, 19 Apr 2017 20:37:38 +0100
From:   Matt Fleming <matt@...eblueprint.co.uk>
To:     Daniel Kiper <daniel.kiper@...cle.com>
Cc:     Mark Rutland <mark.rutland@....com>,
        Julien Grall <julien.grall@....com>,
        Juergen Gross <jgross@...e.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        sstabellini@...nel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, xen-devel@...ts.xen.org,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        linux-efi@...r.kernel.org
Subject: Re: [PATCH] arm64: xen: Implement EFI reset_system callback

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().