Re: [PATCH v4 2/4] xen/arm: Handle cases when hardware_domain is NULL

From: Stefano Stabellini <sstabellini@kernel.org>
To: Luca Fancellu <luca.fancellu@arm.com>
Cc: Julien Grall <julien@xen.org>,
	xen-devel@lists.xenproject.org,
	 Bertrand Marquis <bertrand.marquis@arm.com>,
	wei.chen@arm.com,  Stefano Stabellini <sstabellini@kernel.org>,
	 Volodymyr Babchuk <Volodymyr_Babchuk@epam.com>,
	 Rahul Singh <rahul.singh@arm.com>
Subject: Re: [PATCH v4 2/4] xen/arm: Handle cases when hardware_domain is NULL
Date: Wed, 14 Apr 2021 13:35:51 -0700 (PDT)	[thread overview]
Message-ID: <alpine.DEB.2.21.2104141334270.4885@sstabellini-ThinkPad-T480s> (raw)
In-Reply-To: <F5760FAD-466F-46EC-A3A4-BAC2B5E73BC8@arm.com>

On Wed, 14 Apr 2021, Luca Fancellu wrote:
> > On 14 Apr 2021, at 14:45, Julien Grall <julien@xen.org> wrote:
> > 
> > Hi Luca,
> > 
> > On 14/04/2021 12:29, Luca Fancellu wrote:
> >>> On 14 Apr 2021, at 12:16, Julien Grall <julien@xen.org> wrote:
> >>> 
> >>> Hi Luca,
> >>> 
> >>> On 14/04/2021 10:14, Luca Fancellu wrote:
> >>>> Among the common and arm codebase there are few cases where
> >>>> the hardware_domain variable is checked to see if the current
> >>>> domain is equal to the hardware_domain, change this cases to
> >>>> use is_hardware_domain() function instead. >
> >>>> Signed-off-by: Luca Fancellu <luca.fancellu@arm.com>
> >>>> ---
> >>>> v4 changes:
> >>>> - removed unneeded check for domain NULL from is_hardware_domain
> >>>>   introduced in v3
> >>> 
> >>> After this change, this patch is only avoid to open-code is_hardware_domain(). Although, it adds an extra speculation barrier.
> >>> 
> >>> I am not against the change, however I think the commit message needs to updated to match what the patch is doing.
> >>> 
> >>> Can you propose a new commit message?
> >> Hi Julien,
> >> Yes I agree, what about:
> >> xen/arm: Reinforce use of is_hardware_domain
> >> Among the common and arm codebase there are few cases where
> > 
> > I would drop 'common' because you are only modifying the arm codebase.
> > 
> >> the hardware_domain variable is checked to see if the current
> >> domain is equal to the hardware_domain, change this cases to
> >> use is_hardware_domain() function instead.
> > 
> > 
> >> In the eventuality that hardware_domain is NULL, is_hardware_domain
> >> will return false because an analysis of the common and arm codebase
> >> shows that is_hardware_domain is called always with a non NULL
> >> domain pointer.
> > 
> > This paragraph seems to come out of the blue. I would drop it.
> > 
> > How about:
> > 
> > "
> > There are a few places on Arm where we use pretty much an open-coded version of is_hardware_domain(). The main difference, is the helper will also block speculation (not yet implemented on Arm).
> > 
> > The existing users are not in hot path, so blocking speculation would not hurt when it is implemented. So remove the open-coded version within the arm codebase.
> > "
> > 
> > If you are happy with the commit message, I will commit it the series tomorrow (to give an opportunity to Stefano to review).
> > 
> 
> Hi Julien,
> 
> Yes your version is much better, thank you very much!

It looks great, thanks for your work on this!