Re: [RFC PATCH 1/2] KVM: PPC: Book3S HV: Make virtual processor area registration more robust

[Paul Mackerras <paulus@samba.org>]

On Tue, Jan 17, 2012 at 10:27:26AM +0100, Alexander Graf wrote:

> The thing I was getting at was not the map during the lifetime, but
> the map during registration. Currently we have:
> 
> 1) Set VPA to x
> 2) Assign feature y to VPA
> 3) Use VPA
> 
> 1 and 2 are the slow path, 3 occurs more frequently. So we want 3 to
> be fast. 1 and 2 don't matter that much wrt performance.
> 
> You are currently mapping the VPA at /, which gets you into this
> map/unmap mess trying to free the previous mapping. If you moved the
> map to step 2 and only stored the GPA at step 1, all map+unmap
> operations except for final unmaps would be in one spot, so you
> wouldn't need to construct this big complex state machine.

That might simplify things - I'll try it and see.  The worry with
doing the map/pin at 2 is that if anything goes wrong we no longer
have the opportunity to return an error for the H_REGISTER_VPA call,
so I'll have to at least do some checking in 1, leading to possibly
more code overall.

Paul.