Re: [Qemu-ppc] [Qemu-devel] [PATCH] PPC: Fix large page support in TCG

[Alexander Graf]

On 09.03.2012, at 04:42, David Gibson wrote:

> On Thu, Mar 08, 2012 at 09:24:53AM -0600, Nathan Whitehorn wrote:
>> 
>> On Mar 7, 2012, at 7:25 PM, David Gibson wrote:
>> 
>>> On Sat, Mar 03, 2012 at 10:39:34AM -0600, Nathan Whitehorn wrote:
>>>> Fix large page support in TCG. The old code would overwrite the
>>>> large page table entry with the fake 4 KB
>>>> one generated here whenever the ref/change bits were updated,
>>>> causing it to point to the wrong area of memory. Instead of creating
>>>> a fake PTE, just update the real address at the end.
>>>> 
>>>> Signed-off-by: Nathan Whitehorn <address@hidden>
>>> 
>>> Hrm.  This looks like a cleaner way of handling things, but I don't
>>> really follow what exactly was going wrong in the old way.  Can you
>>> spell out in more detail where the modified pte1 value caused
>>> problems?
>> 
>> The problem was that pte1 would get extra bits added into it in
>> _find_pte() to produce a new, fake 4KB page table entry. When the
>> ref/change bits were updated, pte1 would be written back to the page
>> table -- *including* the bits added to make a fake 4K page. At the
>> next access, since this function does not clear the low bits of
>> large pages (which is probably itself a bug) when it interprets
>> them, the generated address would be the large page base, ored with
>> the large page remainder for this access, ored with the large page
>> remainder from the *previous* access, etc. and you would get a
>> progressively more bogus address in the end.
> 
> Ah, yes, I see it now.  Good catch.
> 
> Acked-by: David Gibson <address@hidden>

Hrm - the patch doesn't apply for me. Could you please resend as something 
that's applyable? :)
Also, please make sure to always CC qemu-ppc on ppc patches, otherwise there's 
a good chance they slip off my radar.


Alex