linux-kernel - Re: [PATCH][EFI] Run EFI in physical mode

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Message-ID: <4C69CFC4.8020307@zytor.com>
Date:	Mon, 16 Aug 2010 16:54:44 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
CC:	huang ying <huang.ying.caritas@...il.com>,
	Takao Indoh <indou.takao@...fujitsu.com>,
	linux-kernel@...r.kernel.org, kexec@...ts.infradead.org,
	tglx@...utronix.de, mingo@...hat.com, vgoyal@...hat.com,
	nhorman@...driver.com
Subject: Re: [PATCH][EFI] Run EFI in physical mode

On 08/16/2010 04:39 PM, Eric W. Biederman wrote:
> 
> "H. Peter Anvin" <hpa@...or.com> writes:
>> "huang ying" <huang.ying.caritas@...il.com> wrote:
>>
>>> On Mon, Aug 16, 2010 at 11:27 AM, H. Peter Anvin <hpa@...or.com> wrote:
>>>> No, it should not be dynamic; rather we should unify all the users
>>>> who need a 1:1 map and just keep that page table set around.
> 
> We still want to restore cr3 from the local task structure as soon
> as is reasonable, as an identity mapped page table will have page 0
> mapped and thus hide null pointer dereferences.
> 

Absolutely!

>>> Agree. One known issue of global 1:1 map is that we need to make at
>>> least part of page table PAGE_KERNEL_EXEC for EFI runtime code, and
>>> change_page_attr can not be used before page allocator is available.
> 
>> For the 1:1 map we probably should make all pages executable; other
>> things need it too, but we shouldn't have it mapped in except when
>> needed.
> 
> We need to be careful in the setup of the global page table so that
> we are in sync with the pat structure for the attributes pages
> are mapped so that we don't map a page as cached and uncached
> at the same time.  Otherwise we could accidentally get cache
> corruption.  To do that would seem to mean change_page_attr
> is relevant at least after we switch from our default set of
> page permissions.

Quite, which is yet another reason to have a common global page table
for all the 1:1 users... right now this is all ad hoc.

	-hpa
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