Re: [Techteam] [RFC PATCH] x86-32: Start out eflags and cr4 clean

[Mitch Bradley]

On 1/18/2013 4:35 PM, H. Peter Anvin wrote:
> On 01/18/2013 05:05 PM, Mitch Bradley wrote:
>>
>>
>> On 1/18/2013 2:42 PM, H. Peter Anvin wrote:
>>> On 01/18/2013 04:40 PM, Andres Salomon wrote:
>>>> Bad news on this patch; I've been told that it breaks booting on an
>>>> XO-1.5.  Does anyone from OLPC know why yet?
>>>
>>> What are the settings of CR0 and CR4 on kernel entry on XO-1.5?
>>
>>
>> CR0 is 0x80000011
>> CR4 is 0x10
>>
> 
> OK, that makes sense... the kernel doesn't enable the PSE bit yet and I
> bet that's what you're using for the non-stolen page tables.

Indeed, we are using 4M pages to map the firmware into high virtual
addresses.

> 
> Can we simply disable paging before mucking with CR4?  The other option
> that I can see is to always enable PSE and PGE, since they are simply
> features opt-ins that don't do any harm if unused.  At the same time,
> though, entering the kernel through the default_entry path with paging
> enabled is definitely not anything the kernel expects.
> 
> Does this patch work for you?  Since we have ditched 386 support, we can
> mimic x86-64 (yay, one more difference gone!) and just use a predefined
> value for %cr0 (the FPU flags need to change if we are on an FPU-less
> chip, but that happens during FPU probing.)
> 
> Does this patch work for you?

We will test it and get back to you.

> 
> 	-hpa
> 
> 
> 
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