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

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Message-ID: <4C1BD053.3050409@zytor.com>
Date:	Fri, 18 Jun 2010 13:00:19 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Ingo Molnar <mingo@...e.hu>
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)

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.

	-hpa
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