Re: [PATCH v3 2/2] kvm: arm64: set io memory s2 pte as normalnc for vfio pci devices

[Oliver Upton]

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