Re: [PATCH 2/2] kvm: powerpc: set cache coherency only for kernel managed pages

[Gleb Natapov <gleb@redhat.com>]

On Wed, Jul 24, 2013 at 12:09:42PM +0200, Alexander Graf wrote:
> 
> On 24.07.2013, at 12:01, Gleb Natapov wrote:
> 
> > Copying Andrea for him to verify that I am not talking nonsense :)
> > 
> > On Wed, Jul 24, 2013 at 10:25:20AM +0200, Alexander Graf wrote:
> >>> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> >>> index 1580dd4..5e8635b 100644
> >>> --- a/virt/kvm/kvm_main.c
> >>> +++ b/virt/kvm/kvm_main.c
> >>> @@ -102,6 +102,10 @@ static bool largepages_enabled = true;
> >>> 
> >>> bool kvm_is_mmio_pfn(pfn_t pfn)
> >>> {
> >>> +#ifdef CONFIG_MEMORY_HOTPLUG
> >> 
> >> I'd feel safer if we narrow this down to e500.
> >> 
> >>> +       /*
> >>> +        * Currently only in memory hot remove case we may still need this.
> >>> +        */
> >>>       if (pfn_valid(pfn)) {
> >> 
> >> We still have to check for pfn_valid, no? So the #ifdef should be down here.
> >> 
> >>>               int reserved;
> >>>               struct page *tail = pfn_to_page(pfn);
> >>> @@ -124,6 +128,7 @@ bool kvm_is_mmio_pfn(pfn_t pfn)
> >>>               }
> >>>               return PageReserved(tail);
> >>>       }
> >>> +#endif
> >>> 
> >>>       return true;
> >>> }
> >>> 
> >>> Before apply this change:
> >>> 
> >>> real    (1m19.954s + 1m20.918s + 1m22.740s + 1m21.146s + 1m22.120s)/5= 1m21.376s
> >>> user    (0m23.181s + 0m23.550s + 0m23.506s + 0m23.410s + 0m23.520s)/5= 0m23.433s
> >>> sys	(0m49.087s + 0m49.563s + 0m51.758s + 0m50.290s + 0m51.047s)/5= 0m50.349s
> >>> 
> >>> After apply this change:
> >>> 
> >>> real    (1m19.507s + 1m20.919s + 1m21.436s + 1m21.179s + 1m20.293s)/5= 1m20.667s
> >>> user    (0m22.595s + 0m22.719s + 0m22.484s + 0m22.811s + 0m22.467s)/5= 0m22.615s
> >>> sys	(0m48.841s + 0m49.929s + 0m50.310s + 0m49.813s + 0m48.587s)/5= 0m49.496s
> >>> 
> >>> So,
> >>> 
> >>> real    (1m20.667s - 1m21.376s)/1m21.376s x 100% = -0.6%
> >>> user    (0m22.615s - 0m23.433s)/0m23.433s x 100% = -3.5%
> >>> sys	(0m49.496s - 0m50.349s)/0m50.349s x 100% = -1.7%
> >> 
> >> Very nice, so there is a real world performance benefit to doing this. Then yes, I think it would make sense to change the global helper function to be fast on e500 and use that one from e500_shadow_mas2_attrib() instead.
> >> 
> >> Gleb, Paolo, any hard feelings?
> >> 
> > I do not see how can we break the function in such a way and get
> > away with it. Not all valid pfns point to memory. Physical address can
> > be sparse (due to PCI hole, framebuffer or just because).
> 
> But we don't check for sparseness today in here either. We merely check for incomplete huge pages.
> 
That's not how I read the code. The code checks for reserved flag set.
It should be set on pfns that point to memory holes. As far as I
understand huge page tricks they are there to guaranty that THP does not
change flags under us, Andrea?

--
			Gleb.