Re: [PATCH v2 1/1] KVM: arm64: allow the VM to select DEVICE_* and NORMAL_NC for IO memory

[Catalin Marinas]

On Wed, Dec 06, 2023 at 11:16:03AM -0400, Jason Gunthorpe wrote:
> On Wed, Dec 06, 2023 at 12:14:18PM +0000, Catalin Marinas wrote:
> > We could do with a pgprot_maybewritecombine() or
> > pgprot_writecombinenospec() (similar to Jason's idea but without
> > changing the semantics of pgprot_device()). For the user mapping on
> > arm64 this would be Device (even _GRE) since it can't disable
> > speculation but stage 2 would leave the decision to the guest since the
> > speculative loads aren't much different from committed loads done
> > wrongly.
> 
> This would be fine, as would a VMA flag. Please pick one :)
> 
> I think a VMA flag is simpler than messing with pgprot.

I guess one could write a patch and see how it goes ;).

> > If we want the VMM to drive this entirely, we could add a new mmap()
> > flag like MAP_WRITECOMBINE or PROT_WRITECOMBINE. They do feel a bit
> 
> As in the other thread, we cannot unconditionally map NORMAL_NC into
> the VMM.

I'm not suggesting this but rather the VMM map portions of the BAR with
either Device or Normal-NC, concatenate them (MAP_FIXED) and pass this
range as a memory slot (or multiple if a slot doesn't allow multiple
vmas).

> > The latter has some benefits for DPDK but it's a lot more involved
> > with
> 
> DPDK WC support will be solved with some VFIO-only change if anyone
> ever cares to make it, if that is what you mean.

Yeah. Some arguments I've heard in private and public discussions is
that the KVM device pass-through shouldn't be different from the DPDK
case. So fixing that would cover KVM as well, though we'd need
additional logic in the VMM. BenH had a short talk at Plumbers around
this - https://youtu.be/QLvN3KXCn0k?t=7010. There was some statement in
there that for x86, the guests are allowed to do WC without other KVM
restrictions (not sure whether that's the case, not familiar with it).

> > having to add device-specific knowledge into the VMM. The VMM would also
> > have to present the whole BAR contiguously to the guest even if there
> > are different mapping attributes within the range. So a lot of MAP_FIXED
> > uses. I'd rather leaving this decision with the guest than the VMM, it
> > looks like more hassle to create those mappings. The VMM or the VFIO
> > could only state write-combine and speculation allowed.
> 
> We talked about this already, the guest must decide, the VMM doesn't
> have the information to pre-predict which pages the guest will want to
> use WC on.

Are the Device/Normal offsets within a BAR fixed, documented in e.g. the
spec or this is something configurable via some MMIO that the guest
does.

-- 
Catalin