On 10/31/19 10:34 AM, Jason Gunthorpe wrote:
On Thu, Oct 31, 2019 at 10:28:12AM -0700, Ralph Campbell wrote:
It seems especially over-complicated to use a full page table layout
for this, wouldn't something simple like an xarray be good enough for
test purposes?
Possibly. A page table is really just a lookup table from virtual address
to pfn/page. Part of the rationale was to mimic what a real device
might do.
Well, but the details of the page table layout don't see really
important to this testing, IMHO.
One problem with XArray is that on 32-bit machines the value would
need to be u64 to hold a pfn which won't fit in a ULONG_MAX.
I guess we could make the driver 64-bit only.
Why would a 32 bit machine need a 64 bit pfn?
On x86, Physical Address Extension (PAE) uses a 64 bit PTE.
See arch/x86/include/asm/pgtable_32_types.h which includes
arch/x86/include/asm/pgtable-3level_types.h.
That is the content of the PTE, not the address of the PTE. In this
case the xarray index is the 'virtual' address of the fictional device
and it can easily be 32 bits with no problem
Jason
Oh, I see. You mean use a 32-bit user virtual address for the index
and store a pointer to the 64-bit PTE which of course would be
32 bit. That should work.
I was stuck on thinking the PTE needed to be stored.
