On Wed, Jul 24, 2013 at 03:32:49PM -0500, Scott Wood wrote: > On 07/24/2013 04:39:59 AM, Alexander Graf wrote: > > > >On 24.07.2013, at 11:35, Gleb Natapov wrote: > > > >> On Wed, Jul 24, 2013 at 11:21:11AM +0200, Alexander Graf wrote: > >>>> Are not we going to use page_is_ram() from > >e500_shadow_mas2_attrib() as Scott commented? > >>> > >>> rWhy aren't we using page_is_ram() in kvm_is_mmio_pfn()? > >>> > >>> > >> Because it is much slower and, IIRC, actually used to build pfn > >map that allow > >> us to check quickly for valid pfn. > > > >Then why should we use page_is_ram()? :) > > > >I really don't want the e500 code to diverge too much from what > >the rest of the kvm code is doing. > > I don't understand "actually used to build pfn map...". What code > is this? I don't see any calls to page_is_ram() in the KVM code, or > in generic mm code. Is this a statement about what x86 does? It may be not page_is_ram() directly, but the same into page_is_ram() is using. On power both page_is_ram() and do_init_bootmem() walks some kind of memblock_region data structure. What important is that pfn_valid() does not mean that there is a memory behind page structure. See Andrea's reply. > > On PPC page_is_ram() is only called (AFAICT) for determining what > attributes to set on mmaps. We want to be sure that KVM always > makes the same decision. While pfn_valid() seems like it should be > equivalent, it's not obvious from the PPC code that it is. > Again pfn_valid() is not enough. > If pfn_valid() is better, why is that not used for mmap? Why are > there two different names for the same thing? > They are not the same thing. page_is_ram() tells you if phys address is ram backed. pfn_valid() tells you if there is struct page behind the pfn. PageReserved() tells if you a pfn is marked as reserved. All non ram pfns should be reserved, but ram pfns can be reserved too. Again, see Andrea's reply. Why ppc uses page_is_ram() for mmap? How should I know? But looking at the function it does it only as a fallback if ppc_md.phys_mem_access_prot() is not provided. Making access to MMIO noncached as a safe fallback makes sense. It is also make sense to allow noncached access to reserved ram sometimes. -- Gleb. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html