Re: [PATCH v2 2/2] KVM: PPC: Book3S HV: Add a per vcpu cache for recently page faulted MMIO entries

[Paul Mackerras <paulus@ozlabs.org>]

On Fri, Nov 04, 2016 at 01:55:12PM +0800, Yongji Xie wrote:
> This keeps a per vcpu cache for recently page faulted MMIO entries.
> On a page fault, if the entry exists in the cache, we can avoid some
> time-consuming paths, for example, looking up HPT, locking HPTE twice
> and searching mmio gfn from memslots, then directly call
> kvmppc_hv_emulate_mmio().
> 
> In current implenment, we limit the size of cache to four. We think
> it's enough to cover the high-frequency MMIO HPTEs in most case.
> For example, considering the case of using virtio device, for virtio
> legacy devices, one HPTE could handle notifications from up to
> 1024 (64K page / 64 byte Port IO register) devices, so one cache entry
> is enough; for virtio modern devices, we always need one HPTE to handle
> notification for each device because modern device would use a 8M MMIO
> register to notify host instead of Port IO register, typically the
> system's configuration should not exceed four virtio devices per
> vcpu, four cache entry is also enough in this case. Of course, if needed,
> we could also modify the macro to a module parameter in the future.
> 
> Signed-off-by: Yongji Xie <xyjxie@linux.vnet.ibm.com>

Thanks, applied to kvm-ppc-next.

Paul.