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This is=0A= >> +=A0=A0=A0=A0=A0 *=A0 only possible if the memory is already mapped into= the kernel map.=0A= >> +=A0=A0=A0=A0=A0 *=0A= >> +=A0=A0=A0=A0=A0 *=A0 Outright reject as the cacheable device memory is = not present in=0A= >> +=A0=A0=A0=A0=A0 *=A0 the kernel map and not suitable for cache manageme= nt.=0A= >> +=A0=A0=A0=A0=A0 */=0A= >> +=A0=A0=A0=A0 if (cacheable_devmem && !stage2_has_fwb(pgt)) {=0A= >> +=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0 ret =3D -EINVAL;=0A= >> +=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0 goto out_unlock;=0A= >> +=A0=A0=A0=A0 }=0A= >> +=0A= >=0A= > These new error reasons should at least be complemented by an=0A= > equivalent check at the point where the memslot is registered. It=0A= =0A= Understood. I can add such check in kvm_arch_prepare_memory_region().=0A= =0A= =0A= > maybe OK to blindly return an error at fault time (because userspace=0A= > has messed with the mapping behind our back), but there should at=0A= > least be something telling a well behaved userspace that there is a=0A= > bunch of combination we're unwilling to support.=0A= =0A= How about WARN_ON() or BUG() for the faulty situation?=0A= =0A= =0A= > Which brings me to the next point: FWB is not discoverable from=0A= > userspace. How do you expect a VMM to know what it can or cannot do?=0A= =0A= Good point. I am not sure if it can. I suppose you are concerned about erro= r=0A= during fault handling when !FWB without VMM having any clear indications=0A= of the cause? Perhaps we can gracefully fall back to the default device map= ping=0A= in such case? But that would cause VM to crash as soon as it makes some=0A= access violating DEVICE_nGnRE.=0A= =0A= >=A0=A0=A0=A0=A0=A0=A0 M.=0A= >=0A= > --=0A= > Without deviation from the norm, progress is not possible.=0A=