linux-kernel - Re: [PATCH] x86: OLPC: add support for calling into OpenFirmware (v5)

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Date:	Sat, 19 Jun 2010 09:43:35 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Andres Salomon <dilinger@...ued.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, tglx@...utronix.de, mingo@...hat.com,
	x86@...nel.org, Mitch Bradley <wmb@...top.org>
Subject: Re: [PATCH] x86: OLPC: add support for calling into OpenFirmware (v5)


* H. Peter Anvin <hpa@...or.com> wrote:

> On 06/18/2010 12:56 PM, Ingo Molnar wrote:
> > 
> > * H. Peter Anvin <hpa@...or.com> wrote:
> > 
> >> On 06/18/2010 12:42 PM, Andres Salomon wrote:
> >>> +
> >>> +	/* install OFW's PDE permanently into the kernel's pgtable */
> >>> +	set_pgd(&swapper_pg_dir[OLPC_OFW_PDE_NR], *ofw_pde);
> >>> +	flush_tlb();
> >>> +	early_iounmap(base, sizeof(olpc_ofw_pgd) * PTRS_PER_PGD);
> >>
> >> I just realized... this flush_tlb() is actually not necessary since on x86 
> >> it is always legal to go from a less permissive configuration to a more 
> >> permissive, and the initial configuration is "not present" which is 
> >> maximally nonpermissive.
> >>
> >> Could you take it out and make sure it still works?
> > 
> > small nit: an optimization barrier would still be needed if the result is 
> > being relied on by the kernel.
> > 
> 
> In this case the optimization barrier is implicit at the return from an
> out of line function.

Which non-obvious side-effect at minimum deserves a comment. (but yes, i'm 
just nitpicking)

Thanks,

	Ingo
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