Re: [RFC PATCH] KVM: PPC: Book3S HV: add support for page faults in VM_IO|VM_PFNMAP vmas

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

On Fri, 2018-02-09 at 16:01 +0100, Paolo Bonzini wrote:
> > > I don't think the else case is useful. If fact you are checking
> > > VM_PFNMAP twice...
> > 
> > yes. I am not sure what the VM_PFNMAP case does. The code comes 
> > from kvmppc_book3s_hv_page_fault() I suppose.
> 
> Maybe the outer check should be for VM_IO|VM_PFNMAP?
> 
> (Also, why is this code reinventing most of hva_to_pfn?  Not asking you
> to fix that of course, but perhaps there's something to improve in the
> generic code too).

I've been wondering the same thing, which led me to try to understand
hva_to_pfn, which resulted in skull marks in the wall ;-)

So hva_to_pfn does a complicated dance around 3 or 4 different variants
of gup and it's really not obvious why, the "intent" doesn't appear to
be documented clearly. I think I somewhat figued out it relates to the
async page fault stuff but even then I don't actually know what those
are and what is their purpose :-)

But yeah, Paul's code more/less does its own thing with a simple
gup_fast, and is missing the hva_to_pfn_remapped case for VM_IO |
VM_PFNMAP. The short fix is to make hva_to_pfn_remapped exported and
use it via a fixed variant of Cedric patch.

The longer term fix might be to understand if/how we can use hva_to_pfn

Cheers,
Ben.