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

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Message-ID: <20170419194329.GP16658@olila.local.net-space.pl>
Date:   Wed, 19 Apr 2017 21:43:29 +0200
From:   Daniel Kiper <daniel.kiper@...cle.com>
To:     Matt Fleming <matt@...eblueprint.co.uk>
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, Apr 19, 2017 at 08:37:38PM +0100, Matt Fleming wrote:
> 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().

This is more or less what I expected. Thanks a lot for explanation.

Daniel