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That lead to the IOMMU being enabled > >>>>> without reserved regions in the low 1MB identity mapped in the p2m for PVH > >>>>> hardware domains. Firmware which happens to be missing RMRR/IVMD ranges > >>>>> describing E820 reserved regions in the low 1MB would transiently trigger IOMMU > >>>>> faults until the p2m is populated by the PVH dom0 builder: > >>>>> > >>>>> AMD-Vi: IO_PAGE_FAULT: 0000:00:13.1 d0 addr 00000000000eb380 flags 0x20 RW > >>>>> AMD-Vi: IO_PAGE_FAULT: 0000:00:13.1 d0 addr 00000000000eb340 flags 0 > >>>>> AMD-Vi: IO_PAGE_FAULT: 0000:00:13.2 d0 addr 00000000000ea1c0 flags 0 > >>>>> AMD-Vi: IO_PAGE_FAULT: 0000:00:14.5 d0 addr 00000000000eb480 flags 0x20 RW > >>>>> AMD-Vi: IO_PAGE_FAULT: 0000:00:12.0 d0 addr 00000000000eb080 flags 0x20 RW > >>>>> AMD-Vi: IO_PAGE_FAULT: 0000:00:14.5 d0 addr 00000000000eb400 flags 0 > >>>>> AMD-Vi: IO_PAGE_FAULT: 0000:00:12.0 d0 addr 00000000000eb040 flags 0 > >>>>> > >>>>> Those errors have been observed on the osstest pinot{0,1} boxes (AMD Fam15h > >>>>> Opteron(tm) Processor 3350 HE). > >>>>> > >>>>> Mostly remove the special handling of the low 1MB done by the PVH dom0 builder, > >>>>> leaving just the data copy between RAM regions. Otherwise rely on the IOMMU > >>>>> arch init code to create any identity mappings for reserved regions in that > >>>>> range (like it already does for reserved regions elsewhere). > >>>>> > >>>>> Note there's a small difference in behavior, as holes in the low 1MB will no > >>>>> longer be identity mapped to the p2m. > >>>> > >>>> I certainly like the simplification, but I'm concerned by this: The BDA > >>>> is not normally reserved, yet may want accessing by Dom0 (to see the real > >>>> machine contents). We do access that first page of memory ourselves, so > >>>> I expect OSes may do so as well (even if the specific aspect I'm thinking > >>>> of - the warm/cold reboot field - is under Xen's control). > >>> > >>> The BDA on the systems I've checked falls into a RAM area on the > >>> memory map, but if you think it can be problematic I could arrange for > >>> arch_iommu_hwdom_init() to also identity map holes in the low 1MB. > >> > >> Hmm, this again is a case where I'd wish CPU and IOMMU mappings could > >> be different. I don't see reasons to try I/O to such holes, but I can > >> see reasons for CPU accesses (of more or less probing kind). > > > > Hm, while I agree devices have likely no reason to access holes (there > > or elsewhere) I don't see much benefit of having this differentiation, > > it's easier to just map everything for accesses from both device and > > CPU rather than us having to decide (and maybe get wrong) whether > > ranges should only be accessed by the CPU. > > I understand that, and I also follow Andrew's arguments towards not > making such a distinction. The consequence though is that we need > to map more than possibly necessary, and never too little. > > >>> Keep in mind this is only for PVH, it won't affect PV. > >> > >> Of course. > > > > Would you be willing to Ack it? > > If "it" is the present version, then me doing so would be stretch. > How averse are you to re-adding the hole mappings? Given the point we are regarding the release I guess it's safer to leave the mapping of the holes in the low 1MB as-is, and consider removing it for 4.19? That would give us a full release cycle to check whether it causes issues on systems. I will send the updated patch. Thanks, Roger.