[Qemu-devel] Re: RFC: New API for PPC for vcpu mmu access

[Alexander Graf]

On 11.02.2011, at 21:53, Scott Wood wrote:

> On Fri, 11 Feb 2011 02:41:35 +0100
> Alexander Graf <address@hidden> wrote:
> 
>>>> Maybe we should go with Avi's proposal after all and simply keep the full 
>>>> soft-mmu synced between kernel and user space? That way we only need a 
>>>> setup call at first, no copying in between and simply update the user 
>>>> space version whenever something changes in the guest. We need to store 
>>>> the TLB's contents off somewhere anyways, so all we need is an additional 
>>>> in-kernel array with internal translation data, but that can be separate 
>>>> from the guest visible data, right?
> 
> Hmm, the idea is growing on me.
> 
>> So then everything we need to get all the functionality we need is a hint 
>> from kernel to user space that something changed and vice versa.
>> 
>> From kernel to user space is simple. We can just document that after every 
>> RUN, all fields can be modified.
>> From user space to kernel, we could modify the entries directly and then 
>> pass in an ioctl that passes in a dirty bitmap to kernel space. KVM can then 
>> decide what to do with it. I guess the easiest implementation for now would 
>> be to ignore the bitmap and simply flush the shadow tlb.
>> 
>> That gives us the flush almost for free. All we need to do is set the tlb to 
>> all zeros (should be done by env init anyways) and pass in the "something 
>> changed" call. KVM can then decide to simply drop all of its shadow state or 
>> loop through every shadow entry and flush it individually. Maybe we should 
>> give a hint on the amount of flushes, so KVM can implement some threshold.
> 
> OK.  We'll also need a config ioctl to specify MMU type/size and the address
> of the arrays.

Right, a setup call basically :).


Alex