Subject: Re: [PATCH] powerpc: Prepare to support kernel handling of IOMMU map/unmap

From: Alexey Kardashevskiy <[email-masked]>
Date: Thu, 25 Jul 2013 20:33:24 +1000

On 07/25/2013 08:26 PM, Alexey Kardashevskiy wrote:
> The current VFIO-on-POWER implementation supports only user mode
> driven mapping, i.e. QEMU is sending requests to map/unmap pages.
> However this approach is really slow, so we want to move that to KVM.
> Since H_PUT_TCE can be extremely performance sensitive (especially with
> network adapters where each packet needs to be mapped/unmapped) we chose
> to implement that as a "fast" hypercall directly in "real
> mode" (processor still in the guest context but MMU off).
> 
> To be able to do that, we need to provide some facilities to
> access the struct page count within that real mode environment as things
> like the sparsemem vmemmap mappings aren't accessible.
> 
> This adds an API to get page struct when MMU is off.
> 
> This adds to MM a new function put_page_unless_one() which drops a page
> if counter is bigger than 1. It is going to be used when MMU is off
> (real mode on PPC64 is the first user) and we want to make sure that page
> release will not happen in real mode as it may crash the kernel in
> a horrible way.

Yes, my english needs to be polished, I even know where :)

-- 
Alexey