netdev - Re: [PATCH 4/8] xen: Use the correctly the Xen memory terminologies

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Date:	Wed, 29 Jul 2015 10:51:18 -0400
From:	Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:	Julien Grall <julien.grall@...rix.com>,
	xen-devel@...ts.xenproject.org
CC:	linux-fbdev@...r.kernel.org, "H. Peter Anvin" <hpa@...or.com>,
	Jiri Slaby <jslaby@...e.com>, stefano.stabellini@...citrix.com,
	Russell King <linux@....linux.org.uk>,
	linux-scsi@...r.kernel.org, x86@...nel.org,
	Tomi Valkeinen <tomi.valkeinen@...com>,
	linux-input@...r.kernel.org,
	Jean-Christophe Plagniol-Villard <plagnioj@...osoft.com>,
	ian.campbell@...rix.com,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	"James E.J. Bottomley" <JBottomley@...n.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	linux-arm-kernel@...ts.infradead.org,
	Juergen Gross <jgross@...e.com>, Wei Liu <wei.liu2@...rix.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	linux-kernel@...r.kernel.org,
	David Vrabel <david.vrabel@...rix.com>, netdev@...r.kernel.org,
	linuxppc-dev@...ts.ozlabs.org,
	Roger Pau Monné <roger.pau@...rix.com>
Subject: Re: [PATCH 4/8] xen: Use the correctly the Xen memory terminologies

On 07/29/2015 10:23 AM, Julien Grall wrote:
> On 29/07/15 15:14, Boris Ostrovsky wrote:
>>> static inline unsigned long pfn_to_gfn(unsigned long pfn)
>>> {
>>>      if (xen_feature(XENFEAT_autotranslated_physmap))
>>>          return pfn;
>>>      else
>>>          return pfn_to_mfn(pfn);
>>> }
>>
>> But you'd still say 'op.arg1.mfn = pfn_to_gfn(pfn);' in xen_do_pin()
>> i.e. assign GFN to MFN, right? That's what I was referring to.
> Well no. I would use op.arg1.mfn = pfn_to_mfn(pfn) given that the code,
> if I'm right, is only executed for PV.
>
> mfn = pfn_to_gfn(...) was valid too because on PV is always an MFN. The
> suggestion of pfn_to_mfn was just for more readability,

Right, and my comments were also not about correctness.


>
>> (In general, I am not sure a guest should ever use 'mfn' as it is purely
>> a hypervisor construct. Including p2m, which I think should really be
>> p2g as this is what we use to figure out what to stick into page tables)
> I think avoid to use mfn in the hypervisor interface is out-of-scope for
> this series. If we ever want to modify the Xen API in Linux, we should
> do in sync with Xen to avoid inconsistency on naming.
>
> Anyway, the oddity of mfn = pfn_to_gfn(...) is mostly contained in the
> x86 specific code. I don't mind to either add pfn_to_mfn and use it or
> add a comment /* PV-specific so mfn == gfn */ for every use of mfn =
> pfn_to_gfn(...).

I think the former is better (even thought it adds a test)

-boris
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