Re: [PATCH v2 1/2][4.15] tools/x86: don't rebuild cpuid-autogen.h every time

From: Jan Beulich <jbeulich@suse.com>
To: "Roger Pau Monné" <roger.pau@citrix.com>
Cc: "xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>,
	Ian Jackson <iwj@xenproject.org>,
	Andrew Cooper <andrew.cooper3@citrix.com>, Wei Liu <wl@xen.org>
Subject: Re: [PATCH v2 1/2][4.15] tools/x86: don't rebuild cpuid-autogen.h every time
Date: Fri, 12 Mar 2021 09:45:35 +0100	[thread overview]
Message-ID: <c88981fc-b83b-b3c0-f003-59af72e6065b@suse.com> (raw)
In-Reply-To: <YEo3VzlTRTaBUkqw@Air-de-Roger>

On 11.03.2021 16:29, Roger Pau Monné wrote:
> On Thu, Mar 11, 2021 at 03:40:05PM +0100, Jan Beulich wrote:
>> The first thing the "xen-dir" rule does is delete the entire xen/
>> subtree. Obviously this includes deleting xen/lib/x86/*autogen.h. As a
>> result there's no original version for $(move-if-changed ...) to compare
>> against, and hence the file and all its consumers would get rebuilt
>> every time. Instead only find and delete all the symlinks.
>>
>> Fixes: eddf9559c977 ("libx86: generate cpuid-autogen.h in the libx86 include dir")
>> Signed-off-by: Jan Beulich <jbeulich@suse.com>
>> ---
>> v2: Different approach.
>> ---
>> Ian did suggest to pass -0r to xargs (and -print0 to find), but I
>> couldn't convince myself that these are standard compliant options. We
>> don't use any special characters in file names, so -print0 / -0
>> shouldn't be necessary at all. The stray rm invocation when there is no
>> output from find can be taken care of by passing -f to it.
> 
> Why not use `-exec rm -f {} +` instead? That seems to be part of
> POSIX and is likely nicer than piping to xargs?

I've looked around some more and also remembered an aspect because
of the looking around.

1) xargs has special provisions to not overrun argument space (leaving
a gap of 2048 bytes). Nothing like this is said in the spec for find,
and in fact I recall having had issues when the accumulated arguments
needed splitting. This was on an old distro, but I wanted to mention
it.

2) I've found (old) indications that the {} may be troublesome to
some distros (Solaris was mentioned) and might need quoting. (This
would of course be easy to deal with.)

Jan