RE: [PATCH v4 for-4.14 2/2] pvcalls: Document correctly and explicitely the padding for all arches

From: Paul Durrant <xadimgnik@gmail.com>
To: "'Jürgen Groß'" <jgross@suse.com>,
	"'Julien Grall'" <julien@xen.org>,
	xen-devel@lists.xenproject.org
Cc: 'Stefano Stabellini' <sstabellini@kernel.org>,
	'Wei Liu' <wl@xen.org>,
	'Andrew Cooper' <andrew.cooper3@citrix.com>,
	'Julien Grall' <jgrall@amazon.com>,
	'Ian Jackson' <ian.jackson@eu.citrix.com>,
	'George Dunlap' <george.dunlap@citrix.com>,
	'Jan Beulich' <jbeulich@suse.com>
Subject: RE: [PATCH v4 for-4.14 2/2] pvcalls: Document correctly and explicitely the padding for all arches
Date: Mon, 29 Jun 2020 08:37:49 +0100	[thread overview]
Message-ID: <000001d64de8$31028b10$9307a130$@xen.org> (raw)
In-Reply-To: <82c445c0-f7fc-176f-fdac-386228cc17f5@suse.com>

> -----Original Message-----
> From: Jürgen Groß <jgross@suse.com>
> Sent: 27 June 2020 12:54
> To: Julien Grall <julien@xen.org>; xen-devel@lists.xenproject.org
> Cc: paul@xen.org; Julien Grall <jgrall@amazon.com>; Andrew Cooper <andrew.cooper3@citrix.com>; George
> Dunlap <george.dunlap@citrix.com>; Ian Jackson <ian.jackson@eu.citrix.com>; Jan Beulich
> <jbeulich@suse.com>; Stefano Stabellini <sstabellini@kernel.org>; Wei Liu <wl@xen.org>
> Subject: Re: [PATCH v4 for-4.14 2/2] pvcalls: Document correctly and explicitely the padding for all
> arches
> 
> On 27.06.20 11:55, Julien Grall wrote:
> > From: Julien Grall <jgrall@amazon.com>
> >
> > The specification of pvcalls suggests there is padding for 32-bit x86
> > at the end of most the structure. However, they are not described in
> > in the public header.
> >
> > Because of that all the structures would be 32-bit aligned and not
> > 64-bit aligned for 32-bit x86.
> >
> > For all the other architectures supported (Arm and 64-bit x86), the
> > structure are aligned to 64-bit because they contain uint64_t field.
> > Therefore all the structures contain implicit padding.
> >
> > Given the specification is authoriitative, the padding will the same for
> 
> s/authoriitative/authoritative/
> 
> > the all architectures. The potential breakage of compatibility is ought
> 
> s/the//
> 
> > to be fine as pvcalls is still a tech preview.
> >
> > As an aside, the padding sadly cannot be mandated to be 0 as they are
> > already present. So it is not going to be possible to use the padding
> > for extending a command in the future.
> >
> > Signed-off-by: Julien Grall <jgrall@amazon.com>
> 
> With above fixed:
> 
> Reviewed-by: Juergen Gross <jgross@suse.com>
> 

Release-acked-by: Paul Durrant <paul@xen.org>

> 
> Juergen