Re: [Xen-devel] [PATCH 7/8] x86/setup: simplify handling of initrdidx when no initrd present

From: Jan Beulich <jbeulich@suse.com>
To: Julien Grall <julien@xen.org>, Andrew Cooper <andrew.cooper3@citrix.com>
Cc: "Stefano Stabellini" <sstabellini@kernel.org>,
	"Wei Liu" <wl@xen.org>,
	"Konrad Rzeszutek Wilk" <konrad.wilk@oracle.com>,
	"George Dunlap" <George.Dunlap@eu.citrix.com>,
	"Varad Gautam" <vrd@amazon.de>,
	"Ian Jackson" <ian.jackson@eu.citrix.com>,
	"Hongyan Xia" <hongyxia@amazon.com>,
	xen-devel@lists.xenproject.org,
	"Paul Durrant" <pdurrant@amazon.co.uk>,
	"David Woodhouse" <dwmw2@infradead.org>,
	"Roger Pau Monné" <roger.pau@citrix.com>
Subject: Re: [Xen-devel] [PATCH 7/8] x86/setup: simplify handling of initrdidx when no initrd present
Date: Wed, 26 Feb 2020 09:37:45 +0100	[thread overview]
Message-ID: <ddea05a7-645f-42d5-43f7-ddf008cdb9cb@suse.com> (raw)
In-Reply-To: <863e5864-5aef-3fde-9433-bc4f8bd19eb2@xen.org>

On 26.02.2020 08:13, Julien Grall wrote:
> On 25/02/2020 12:34, Jan Beulich wrote:
>> Further, even if all current implementations obeyed by the more
>> strict rule, this still wouldn't mean callers are actually permitted
>> to assume such. The more strict rule would then also need to be
>> written down, such that it won't get violated again in the future
>> (by changes to an existing arch's implementation, or by a new port
> 
> To be honest, the rule should be written down in any case. The current 
> one is not necessarily an obvious one and also differ from what Linux 
> folks can expect.

I think we should stick to the more relaxed rule in any event, unless
there's a strong reason to enforce the more strict one. Much (but not
all) of Linux code looks to assume the more relaxed rule too, likely
also for historical reasons (when the implementation on e.g. x86-64
still followed the more relaxed model).

> Regarding future port, the number of architectures in Linux using custom 
> bitops are fairly limited (AFAICT only arm32 and unicore32). All the 
> rest (including x86) using a generic implementation.
> 
> On Xen, Arm64 is already using the generic implementation. Is there any 
> particular concern to use it for x86 as well?

According to the (over 10 years old) commit updating Linux x86 this
way, the generic implementation was even faster. If that was the
case today and for our implementation as well, then I think it
would be very nice if we updated. If, otoh, data isn't as clear,
then further consideration may be needed. Andrew, do you have any
thoughts either way?

> If not, I can pull a patch to use the generic implementation on 
> x86/arm32. This would solve the discrenpancies in find_*_bit 
> implementations.

This is orthogonal to the issue discussed - as said, I think code
using the functions would still better assume the more relaxed model.

Jan

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