On 07/25/2013 03:50:42 AM, Gleb Natapov wrote:
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.
There's only one current implementation of ppc_md.phys_mem_access_prot(), which is pci_phys_mem_access_prot(), which also uses page_is_ram(). If page_is_ram() returns false then it checks for write-combining PCI. But yes, we would want to call ppc_md.phys_mem_access_prot() if present.
Copying from the host PTE would be ideal if doesn't come with a noticeable performance impact compared to other methods, but one way or another we want to be sure we match.
It is also make sense to allow noncached access to reserved ram sometimes.
Perhaps, but that's not KVM's decision to make. You should get the same result as if you mmaped it -- because QEMU already did and we need to be consistent. Not to mention the large page kernel mapping that will have been done on e500...
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