Re: RFC: New API for PPC for vcpu mmu access

[Scott Wood <scottwood@freescale.com>]

On Mon, 7 Feb 2011 17:49:51 +0100
Alexander Graf <agraf@suse.de> wrote:

> 
> On 07.02.2011, at 17:40, Yoder Stuart-B08248 wrote:
> 
> > Suggested change to this would be to have Qemu set tlb_type as 
> > an _input_ argument.   If KVM supports it, that type gets used,
> > else an error is returned.    This would allow Qemu to tell
> > the kernel what type of MMU it is prepared to support.   Without
> > this Qemu would just have to error out if the type returned is
> > unknown.
> 
> Yes, we could use the same struct for get and set. On set, it could transfer the mmu type, on get it could tell userspace the mmu type.

What happens if a get is done before the first set, and there are
multiple MMU type options for this hardware, with differing entry sizes?

Qemu would have to know beforehand how large to make the buffer.

We could say that going forward, it's expected that qemu will do a
TLB set (either a full one, or a lightweight alternative) after
creating a vcpu.  For compatibility, if this doesn't happen before the
vcpu is run, the TLB is created and initialized as it is today, but
no new Qemu-visible features will be enabled that way.

If Qemu does a get without ever doing some set operation, it should
get an error, since the requirement to do a set is added at the same
time as the get API.

-Scott