From mboxrd@z Thu Jan  1 00:00:00 1970
From: Andrew Cooper <andrew.cooper3@citrix.com>
Subject: Re: [PATCH 1/3] spinlock: use local_irq_disable()
 instead of local_irq_save() where possible
Date: Fri, 9 Jan 2015 16:09:30 +0000
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To: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>, Jan Beulich <JBeulich@suse.com>
Cc: Ian Campbell <Ian.Campbell@eu.citrix.com>, xen-devel <xen-devel@lists.xenproject.org>, Keir Fraser <keir@xen.org>, Ian Jackson <Ian.Jackson@eu.citrix.com>, Tim Deegan <tim@xen.org>
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On 09/01/15 16:02, Konrad Rzeszutek Wilk wrote:
> On Thu, Jan 08, 2015 at 04:13:03PM +0000, Jan Beulich wrote:
>> ... as generally being a cheaper operation.
> I was wondering if it would be possible to change some of the
> EFLAGS after when we go in the 'cpu_relax' - and an interrupt
> happens, we process it, alter the EFLAGS, then when we are
> done, the EFLAGS are different - which the original code would
> save when it was done sitting on the cpu_relax() loop.
>
> Actually that sounds bad - we only want to restore the flags
> that we had when going in this spin lock. Would make sense
> to add an ASSERT to check for flags being different from the
> EFLAGS?

local_irq_restore() only restores the interrupt flag from flags.  All
other bits in EFLAGS are unmodified.

~Andrew