Re: [PATCH RFC 1/1] Add support for ZONE_DEVICE IO memory with struct pages.

Matthew Wilcox
2016-03-14T21:23:44Z

On Mon, Mar 14, 2016 at 12:14:37PM -0600, Stephen Bates wrote:
> 3. Coherency Issues. When IOMEM is written from both the CPU and a PCIe
> peer there is potential for coherency issues and for writes to occur out
> of order. This is something that users of this feature need to be
> cognizant of and may necessitate the use of CONFIG_EXPERT. Though really,
> this isn't much different than the existing situation with RDMA: if
> userspace sets up an MR for remote use, they need to be careful about
> using that memory region themselves.

There's more to the coherency problem than this. As I understand it, on
x86, memory in a PCI BAR does not participate in the coherency protocol.
So you can get a situation where CPU A stores 4 bytes to offset 8 in a
cacheline, then CPU B stores 4 bytes to offset 16 in the same cacheline,
and CPU A's write mysteriously goes missing.

I may have misunderstood the exact details when this was explained to me a
few years ago, but the details were horrible enough to run away screaming.
Pretending PCI BARs are real memory? Just Say No.