On Fri, Jun 24, 2022 at 01:12:56PM -0700, Nicolin Chen wrote: > > The kmap_local_page() arose because the code doing memcpy had to be > > updated to go from a struct page to a void * for use with memcpy and > > the kmap_local_page() is the correct API to use for that. > > > > The existing code which casts a pfn to a void * is improper. > > Yes. > > If I understand everything correctly: > > A PFN is not secure enough to promise that the memory is not IO. And > direct access via memcpy() that only handles CPU memory will crash on > S390 if the PFN is an IO PFN, as we have to use the memcpy_to/fromio() > that uses the special S390 IO access instructions. On the other hand, > a "struct page *" is always a CPU coherent thing that fits memcpy(). > > Also, casting a PFN to "void *" for memcpy() is not an proper practice, > kmap_local_page() is the correct API to call here, though S390 doesn't > use highmem, which means kmap_local_page() is a NOP. > > There's a following patch changing the vfio_pin_pages() API to return > a list of "struct page *" instead of PFNs. It will block any IO memory > from ever getting into this call path, for such a security purpose. In > this patch, add kmap_local_page() to prepare for that. Yes, basically Jason
