linux-kernel - Re: [PATCH v4 4/7] x86, mm, pat: Add pgprot

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Date:	Mon, 3 Nov 2014 15:01:00 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Toshi Kani <toshi.kani@...com>,
	"Elliott, Robert (Server Storage)" <Elliott@...com>,
	"hpa@...or.com" <hpa@...or.com>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"arnd@...db.de" <arnd@...db.de>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"jgross@...e.com" <jgross@...e.com>,
	"stefan.bader@...onical.com" <stefan.bader@...onical.com>,
	"hmh@....eng.br" <hmh@....eng.br>,
	"yigal@...xistor.com" <yigal@...xistor.com>,
	"konrad.wilk@...cle.com" <konrad.wilk@...cle.com>
Subject: Re: [PATCH v4 4/7] x86, mm, pat: Add pgprot_writethrough() for WT

On Mon, Nov 3, 2014 at 2:53 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
> On Mon, 3 Nov 2014, Toshi Kani wrote:
>> On Mon, 2014-11-03 at 22:10 +0000, Elliott, Robert (Server Storage)
>> wrote:
>>  :
>> > > Subject: [PATCH v4 4/7] x86, mm, pat: Add pgprot_writethrough() for
>> > > WT
>> > >
>> > > This patch adds pgprot_writethrough() for setting WT to a given
>> > > pgprot_t.
>> > >
>> > > Signed-off-by: Toshi Kani <toshi.kani@...com>
>> > > Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
>> > ...
>> > > diff --git a/arch/x86/mm/pat.c b/arch/x86/mm/pat.c
>> > > index a214f5a..a0264d3 100644
>> > > --- a/arch/x86/mm/pat.c
>> > > +++ b/arch/x86/mm/pat.c
>> > > @@ -896,6 +896,16 @@ pgprot_t pgprot_writecombine(pgprot_t prot)
>> > >  }
>> > >  EXPORT_SYMBOL_GPL(pgprot_writecombine);
>> > >
>> > > +pgprot_t pgprot_writethrough(pgprot_t prot)
>> > > +{
>> > > + if (pat_enabled)
>> > > +         return __pgprot(pgprot_val(prot) |
>> > > +                         cachemode2protval(_PAGE_CACHE_MODE_WT));
>> > > + else
>> > > +         return pgprot_noncached(prot);
>> > > +}
>> > > +EXPORT_SYMBOL_GPL(pgprot_writethrough);
>> > ...
>> >
>> > Would you be willing to use EXPORT_SYMBOL for the new
>> > pgprot_writethrough function to provide more flexibility
>> > for modules to utilize the new feature?  In x86/mm, 18 of 60
>> > current exports are GPL and 42 are not GPL.
>>
>> I simply used EXPORT_SYMBOL_GPL() since pgprot_writecombine() used
>> it. :-)  This interface is intended to be used along with
>> remap_pfn_range() and ioremap_prot(), which are both exported with
>> EXPORT_SYMBOL().  So, it seems reasonable to export it with
>> EXPORT_SYMBOL() as well.  I will make this change.
>
> NAK.
>
> This is new functionality and we really have no reason to give the GPL
> circumventors access to it.

I have mixed feelings about this.

On the one hand, I agree with your sentiment.

On the other hand, I thought that _GPL was supposed to be more about
whether the thing using it is inherently a derived work of the Linux
kernel.  Since WT is an Intel concept, not a Linux concept, then I
think that this is a hard argument to make.

Not that I mind encouraging HP to GPL everything.  Although my
experiences so far with HP servers have been so uniformly negative
that I really just want to stay far away from anything storage-related
by HP for several years, so I'm very unlikely to own an affected piece
of hardware any time soon.  (Sorry, HP.)

--Andy
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