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. > 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. > 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. > 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. Jason
