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

[Oliver Upton <oliver.upton@linux.dev>]

On Thu, Dec 14, 2023 at 04:48:15PM +0100, Lorenzo Pieralisi wrote:

[...]

> > 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.
> 
> FWIW - I have tried to summarize the reasoning behind PCIe devices
> Normal-NC default stage-2 safety in a document that I have just realized
> now it has become this series cover letter, I don't think the PCI blanket
> treatment is related *only* to the current user space mappings (ie
> BTW, AFAICS it is also *possible* at present to map a prefetchable BAR through
> sysfs with Normal-NC memory attributes in the host at the same time a PCI
> device is passed-through to a guest with VFIO - and therefore we have a
> dev-nGnRnE stage-1 mapping for it. Don't think anyone does that - what for -
> but it is possible and KVM would not know about it).
> 
> Again, FWIW, we were told (source Arm ARM) mismatched aliases concerning
> device-XXX vs Normal-NC are not problematic as long as the transactions
> issued for the related mappings are independent (and none of the
> mappings is cacheable).
> 
> I appreciate this is not enough to give everyone full confidence on
> this solution robustness - that's why I wrote that up so that we know
> what we are up against and write KVM interfaces accordingly.

Apologies, I didn't mean to question what's going on here from the
hardware POV. My concern was more from the kernel + user interfaces POV,
this all seems to work (specifically for PCI) by maintaining an
intentional mismatch between the VFIO stage-1 and KVM stage-2 mappings.

If we add more behind-the-scenes tricks to get other MMIO mappings
working in the future then this whole interaction will get even
hairier. At least if we follow the stage-1 attributes (where possible)
then we can document some sort of expected behavior in KVM. The VMM would
need know if the device has read side-effects, as the only way to get a
Normal-NC mapping in the guest would be to have one at stage-1.

Kinda stinks to make the VMM aware of the device, but IMO it is a
fundamental limitation of the way we back memslots right now.

-- 
Thanks,
Oliver