Re: [kvm-devel] [RFC/PATCH 01/15] preparation: provide hook to enable pgstes in user pagetable

[Carsten Otte <cotte@de.ibm.com>]

Dave Hansen wrote:
> On Thu, 2008-03-20 at 21:35 +0100, Carsten Otte wrote:
>> Dave Hansen wrote:
>>> Well, and more fundamentally: do we really want dup_mm() able to be
>>> called from other code?
>>>
>>> Maybe we need a bit more detailed justification why fork() itself isn't
>>> good enough.  It looks to me like they basically need an arch-specific
>>> argument to fork, telling the new process's page tables to take the
>>> fancy new bit.
>>>
>>> I'm really curious how this new stuff is going to get used.  Are you
>>> basically replacing fork() when creating kvm guests?
>> No. The trick is, that we do need bigger page tables when running 
>> guests: our page tables are usually 2k, but when running a guest 
>> they're 4k to track both guest and host dirty&reference information. 
>> This looks like this:
>> *----------*
>> *2k PTE's  *
>> *----------*
>> *2k PGSTE  *
>> *----------*
>> We don't want to waste precious memory for all page tables. We'd like 
>> to have one kernel image that runs regular server workload _and_ 
>> guests.
> 
> That makes a lot of sense.
> 
> Is that layout (the shadow and regular stacked together) specified in
> hardware somehow, or was it just chosen?
It's defined by hardware. The chip just adds +2k to the ptep to get to 
the corresponding pgste. Both pte and pgste are 64bit per page. I know 
Heiko and Martin have thought a lot about possible races. I'll have to 
leave your question on the race against pfault open for them.

Btw: thanks a lot for reviewing our changes :-)

cheers,
Carsten