From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andrew Cooper Subject: Re: [PATCH v4] x86/arm/mm: use gfn instead of pfn in p2m_get_mem_access/p2m_set_mem_access Date: Thu, 2 Jul 2015 12:38:11 +0100 Message-ID: <559522A3.6040806@citrix.com> References: <1435592708-26456-1-git-send-email-vkuznets@redhat.com> <559516F9.8050402@eu.citrix.com> <55951BDC.7010002@citrix.com> <20150702112549.GA87948@deinos.phlegethon.org> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20150702112549.GA87948@deinos.phlegethon.org> List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xen.org Errors-To: xen-devel-bounces@lists.xen.org To: Tim Deegan Cc: Keir Fraser , Ian Campbell , Razvan Cojocaru , George Dunlap , xen-devel@lists.xen.org, Stefano Stabellini , Jan Beulich , Tamas K Lengyel , Vitaly Kuznetsov List-Id: xen-devel@lists.xenproject.org On 02/07/15 12:25, Tim Deegan wrote: > At 12:09 +0100 on 02 Jul (1435838956), Andrew Cooper wrote: >> On 02/07/15 11:48, George Dunlap wrote: >>> Now in p2m_set_mem_access(), rather than just using an unsigned long in >>> the loop iterating over gfns, you do this thing where you convert gfn_t >>> to unsigned long, add one, and then convert it back to gfn_t again. >>> >>> I can't see any comments in v3 that suggest you doing that, and it seems >>> a bit clunky. Is that really necessary? Wouldn't it be better to >>> declare a local variable? >>> >>> I'm not strongly opinionated on this one, it just seems a bit strange. >>> >>> Everything else looks good, thanks. >> Looping over {g,m,p}fn_t's is indeed awkward, as the compiler tricks for >> typesafety don't allow for simply adding 1 to a typesafe variable. >> >> In a cases like this, I think it is acceptable to keep a unsigned long >> shadow variable and manipulate it is a plain integer. The eventual >> _gfn() required to pass it further down the callchain will help to >> visually re-enforce the appropriate type. >> >> After all, the entire point of these typesafes are to try and avoid >> accidentally mixing up the different address spaces, but a function >> which takes a typesafe, loops over a subset and passes the same typesafe >> further down can probably be trusted to DTRT, catching errors at review >> time. >> >> Off the top of my head, the only functions which would normally expect >> to mix and match the typesafes are the pagetable walking ones. > It should be easy enough to extend the macros to define a > gfn_inc(&gfn_t) operator for this kind of thing. Very true. I suspect inc and dec will cover the majority of the unbox/modify/box bits we currently have. OTOH, ideally I would prefer not to introduce even more magic which cscope/tags can't spot, but introducing gfn_inc() does appear to be the neater solution. ~Andrew