Re: How can I know Page Table address on RAM?

[Nicholas Piggin]

On Mon Jan 22, 2024 at 6:54 PM AEST, Thomas Huth wrote:
> On 22/01/2024 05.11, Junho wrote:
> > Hello,
> > 
> > I'm a QEMU user with PowerPc target architecture.
> > I have some personal modifications related to tb jmp cache and chaining 
> > logic to improve the performance of a specific guest code. To verify the 
> > safety, I have to guarantee that the page table on RAM does not change 
> > after 
> > initialization. Do you have any information related to this work? 
> > Currently, 
> > what I need to find is the page table start address on the RAM so that I 
> > can 
> > test with the range detected.
> > 
> > I look forward to your response.
> > 
> > Thank you
> > Junho
>
>   Hi,
>
> maybe it's best to ask this question on the qemu-ppc mailing list instead 
> (done now), since most PPC folks will rather read than one instead of the 
> high-traffic qemu-devel mailing list.

Hi Junho,

ppc targets have a lot of different MMUs, so it depends what you are
looking at.

The hash MMU has a page table that is linear in physical (real) memory,
so you might feasibly be able to watch it for updates. The SDR1 SPR has
hash table base and size. ISA v3.0 and later use an in-memory table
that is pointed to by the PTCR SPR.

Other types are software loaded and radix page tables which might be
infeasible or impossible to really track.

It would be interesting to know what kind of modifications you're doing,
it's possible they might be achieved another way. For example, there is
no requirement in the architecture for the TLB to be kept coherent with
page table modifications, so you might be able to watch for TLB flush
instructions rather than page table changes.

Thanks,
Nick