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:
> 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. Therefore, we need to reallocate the page table after fork() 
once we know that task is going to be a hypervisor. That's what this 
code does: reallocate a bigger page table to accomondate the extra 
information. The task needs to be single-threaded when calling for 
extended page tables.

Btw: at fork() time, we cannot tell whether or not the user's going to 
be a hypervisor. Therefore we cannot do this in fork.

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