Re: [PATCH v4 4/7] x86, mm, pat: Add pgprot_writethrough() for WT

[Andy Lutomirski <luto@amacapital.net>]

On Mon, Nov 3, 2014 at 2:53 PM, Thomas Gleixner <tglx@linutronix.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@hp.com>
>> > > Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.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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