Hi,
Sorry, a bit late to the discussion :)
On Tue, Dec 12, 2023 at 02:11:56PM -0400, Jason Gunthorpe wrote:
> On Tue, Dec 12, 2023 at 05:46:34PM +0000, Catalin Marinas wrote:
> > should know the implications. There's also an expectation that the
> > actual driver (KVM guests) or maybe later DPDK can choose the safe
> > non-cacheable or write-combine (Linux terminology) attributes for the
> > BAR.
>
> DPDK won't rely on this interface
Wait, so what's the expected interface for determining the memory
attributes at stage-1? I'm somewhat concerned that we're conflating two
things here:
1) KVM needs to know the memory attributes to use at stage-2, which
isn't fundamentally different from what's needed for userspace
stage-1 mappings.
2) KVM additionally needs a hint that the device / VFIO can handle
mismatched aliases w/o the machine exploding. This goes beyond
supporting Normal-NC mappings at stage-2 and is really a bug
with our current scheme (nGnRnE at stage-1, nGnRE at stage-2).
I was hoping that (1) could be some 'common' plumbing for both userspace
and KVM mappings. And for (2), any case where a device is intolerant of
mismatches && KVM cannot force the memory attributes should be rejected.
AFAICT, the only reason PCI devices can get the blanket treatment of
Normal-NC at stage-2 is because userspace has a Device-* mapping and can't
speculatively load from the alias. This feels a bit hacky, and maybe we
should prioritize an interface for mapping a device into a VM w/o a
valid userspace mapping.
I very much understand that this has been going on for a while, and we
need to do *something* to get passthrough working well for devices that
like 'WC'. I just want to make sure we don't paint ourselves into a corner
that's hard to get out of in the future.
--
Thanks,
Oliver
