Re: [Xen-devel] [PATCH 5/6] iommu: tidy up iommu_us_hap_pt() and need_iommu_pt_sync() macros

From: Paul Durrant <Paul.Durrant@citrix.com>
To: 'Julien Grall' <julien.grall@arm.com>, 'Jan Beulich' <jbeulich@suse.com>
Cc: Stefano Stabellini <sstabellini@kernel.org>, Wei Liu <wl@xen.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
	Andrew Cooper <Andrew.Cooper3@citrix.com>,
	"Tim \(Xen.org\)" <tim@xen.org>,
	George Dunlap <George.Dunlap@citrix.com>,
	Ian Jackson <Ian.Jackson@citrix.com>,
	"xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>,
	Volodymyr Babchuk <Volodymyr_Babchuk@epam.com>,
	Roger Pau Monne <roger.pau@citrix.com>
Subject: Re: [Xen-devel] [PATCH 5/6] iommu: tidy up iommu_us_hap_pt() and need_iommu_pt_sync() macros
Date: Wed, 14 Aug 2019 12:35:57 +0000	[thread overview]
Message-ID: <3e0736f5abb2460a865cc250e9b1b5a9@AMSPEX02CL03.citrite.net> (raw)
In-Reply-To: <bcfe1bab-fe1f-f6ae-52c9-8fa50b878963@arm.com>

> -----Original Message-----
[snip]
> >>>>>>> +/* Are we using the domain P2M table as its IOMMU pagetable? */
> >>>>>>> +#define iommu_use_hap_pt(d) \
> >>>>>>> +    (hap_enabled(d) && is_iommu_enabled(d) && iommu_hap_pt_share)
> >>>>>>
> >>>>>> Does this build for Arm, seeing that there's no hap_enabled()
> >>>>>> definition there? Or have I missed its addition earlier in this
> >>>>>> series?
> >>>>>
> >>>>> It moved to common code sched.h in an earlier patch.
> >>>>
> >>>> I went through the series and didn't find where hap_enabled() is defined for Arm
> >>>> in this series. Do you mind pointing the exact patch?
> >>>>
> >>>
> >>> Sorry, I wasn't clear... The change is in my other series, "use stashed domain create flags",
> which
> >> is a pre-requisite for this series (as called out in the cover letter). The change is made in patch
> #2
> >> of that series: https://lists.xenproject.org/archives/html/xen-devel/2019-07/msg02256.html.
> >>
> >> Oh. I understand this adds benefits as the implementation is now common. But the
> >> downside is hap_enabled() will now require evaluation on Arm even it is
> >> evaluates to true... This will prevent the compiler to remove any non-HAP code
> >> paths (assuming there are any in the common code).
> >
> > There was one in the common iommu code that thus required a #ifdef for ARM.
> 
> Any CONFIG_{ARM, X86} feels an abuse of common code. So I am always in favor of
> dropping them :). My concern is that a few of the helpers will likely return a
> single value for any architecture by x86. But that's a different problem...
> 
> >
> >>
> >> Furthermore, 2 parts of the iommu_use_hap_pt() condition will always returning
> >> always true. But as they are non-constant, so they will always be evaluated.
> >>
> >> It is also probably going to confuse developer as they may think non-HAP is
> >> supported on Arm. You can't find easily that both hap_enabled(...) and
> >> iommu_hap_pt_share will always evaluate to true.
> >>
> >> So aside the common implementation, what is the real gain for Arm?
> >
> > There's no real gain for ARM, the gain is in the reduction in ifdef-ery and thus tidiness of code. I
> could put back some ifdefs if you'd prefer, or I could just put a comment stating that
> iommu_use_hap_pt() will always be true for ARM. Which would you prefer?
> 
> Looking at the patch #6, iommu_use_hap_pt() is reimplemented with:
> 
> #define iommu_use_hap_pt(d)       (dom_iommu(d)->hap_pt_share)
> 
> You also have a comment mentioning Arm systems in the same patch.
> 
> So I would be happy with this patch assuming that patch #6 does not change.
> 

Ok, thanks. I'll see about adding a patch for arch specific defs of things like is_hvm_domain() and is_pv_domain(), and hap_enabled().

  Paul
_______________________________________________
Xen-devel mailing list
Xen-devel@lists.xenproject.org
https://lists.xenproject.org/mailman/listinfo/xen-devel