Re: [PATCH v8 13/15] ARM: KVM: Handle guest faults in KVM

[Andrea Arcangeli <aarcange@redhat.com>]

On Tue, Jun 19, 2012 at 12:32:06PM +0300, Avi Kivity wrote:
> On 06/19/2012 01:20 AM, Christoffer Dall wrote:
> > On Mon, Jun 18, 2012 at 9:45 AM, Avi Kivity <avi@redhat.com> wrote:
> >> On 06/15/2012 10:09 PM, Christoffer Dall wrote:
> >>> From: Christoffer Dall <cdall@cs.columbia.edu>
> >>>
> >>> Handles the guest faults in KVM by mapping in corresponding user pages
> >>> in the 2nd stage page tables.
> >>>
> >>> Introduces new ARM-specific kernel memory types, PAGE_KVM_GUEST and
> >>> pgprot_guest variables used to map 2nd stage memory for KVM guests.
> >>>
> >>> Leverages MMU notifiers on KVM/ARM by supporting the kvm_unmap_hva() operation,
> >>> where we remove the HVA from the 2nd stage translation. All other KVM MMU
> >>> notifierhooks are NOPs.
> >>
> >> I think you must at least support change_pte (possibly by unmapping).
> >> Andrea?
> >>
> > hmmm, at least for KSM support we need to support change_pte (are
> > there other callers for this type of memory?)
> > 
> > It's not trivial I guess, since we would need to support COW and
> > thereby stage-2 permission faults... Marc, right?
> 
> As I mentioned, you can support change_pte by unmapping.  This will
> cause ksm to be ineffective (pages will only be shared if the guest
> doesn't touch them at all), but it's enough to get started.

The main reason change_pte initially was required for KSM to be
effective was because gup_fast was called with write=1
unconditionally. change_pte was also responsible to set the spte
readonly. But that should have been fixed now on x86, so KSM should be
effective even despite lack of change_pte on x86.

If the KVM page fault is calling gfn_to_pfn_async(write=0/1) depending
if the vmexit was caused by a write or read access (instead of
gfn_to_pfn which still has the unconditional write=1), and in turn
it's forced to sete the spte readonly after calling
gfn_to_pfn_async(write=0), change_pte is still useful but it's only a
worthwhile optimization to avoid a spte read fault after every KSM
page merged, it's not strictly required for KSM effectiveness anymore.

In short if ARM does the right thing with regard of KVM read faults
passed to gup_fast(write=0) and setting the spte readonly, all should
work good with KSM (even if not as optimal as with change_pte).