On Tue, Dec 05, 2023 at 09:00:15AM +0530, ankita@xxxxxxxxxx wrote: > From: Ankit Agrawal <ankita@xxxxxxxxxx> > > Currently, KVM for ARM64 maps at stage 2 memory that is considered device > (i.e. it is not RAM) with DEVICE_nGnRE memory attributes; this setting > overrides (as per the ARM architecture [1]) any device MMIO mapping > present at stage 1, resulting in a set-up whereby a guest operating > system cannot determine device MMIO mapping memory attributes on its > own but it is always overridden by the KVM stage 2 default. > > This set-up does not allow guest operating systems to select device > memory attributes independently from KVM stage-2 mappings > (refer to [1], "Combining stage 1 and stage 2 memory type attributes"), > which turns out to be an issue in that guest operating systems > (e.g. Linux) may request to map devices MMIO regions with memory > attributes that guarantee better performance (e.g. gathering > attribute - that for some devices can generate larger PCIe memory > writes TLPs) and specific operations (e.g. unaligned transactions) > such as the NormalNC memory type. > > The default device stage 2 mapping was chosen in KVM for ARM64 since > it was considered safer (i.e. it would not allow guests to trigger > uncontained failures ultimately crashing the machine) but this > turned out to be asynchronous (SError) defeating the purpose. > > Failures containability is a property of the platform and is independent > from the memory type used for MMIO device memory mappings. > > Actually, DEVICE_nGnRE memory type is even more problematic than > Normal-NC memory type in terms of faults containability in that e.g. > aborts triggered on DEVICE_nGnRE loads cannot be made, architecturally, > synchronous (i.e. that would imply that the processor should issue at > most 1 load transaction at a time - it cannot pipeline them - otherwise > the synchronous abort semantics would break the no-speculation attribute > attached to DEVICE_XXX memory). > > This means that regardless of the combined stage1+stage2 mappings a > platform is safe if and only if device transactions cannot trigger > uncontained failures and that in turn relies on platform capabilities > and the device type being assigned (i.e. PCIe AER/DPC error containment > and RAS architecture[3]); therefore the default KVM device stage 2 > memory attributes play no role in making device assignment safer > for a given platform (if the platform design adheres to design > guidelines outlined in [3]) and therefore can be relaxed. > > For all these reasons, relax the KVM stage 2 device memory attributes > from DEVICE_nGnRE to Normal-NC. Add a new kvm_pgtable_prot flag for > Normal-NC. > > The Normal-NC was chosen over a different Normal memory type default > at stage-2 (e.g. Normal Write-through) to avoid cache allocation/snooping. > > Relaxing S2 KVM device MMIO mappings to Normal-NC is not expected to > trigger any issue on guest device reclaim use cases either (i.e. device > MMIO unmap followed by a device reset) at least for PCIe devices, in that > in PCIe a device reset is architected and carried out through PCI config > space transactions that are naturally ordered with respect to MMIO > transactions according to the PCI ordering rules. > > Having Normal-NC S2 default puts guests in control (thanks to > stage1+stage2 combined memory attributes rules [1]) of device MMIO > regions memory mappings, according to the rules described in [1] > and summarized here ([(S1) - stage1], [(S2) - stage 2]): > > S1 | S2 | Result > NORMAL-WB | NORMAL-NC | NORMAL-NC > NORMAL-WT | NORMAL-NC | NORMAL-NC > NORMAL-NC | NORMAL-NC | NORMAL-NC > DEVICE<attr> | NORMAL-NC | DEVICE<attr> > > It is worth noting that currently, to map devices MMIO space to user > space in a device pass-through use case the VFIO framework applies memory > attributes derived from pgprot_noncached() settings applied to VMAs, which > result in device-nGnRnE memory attributes for the stage-1 VMM mappings. > > This means that a userspace mapping for device MMIO space carried > out with the current VFIO framework and a guest OS mapping for the same > MMIO space may result in a mismatched alias as described in [2]. > > Defaulting KVM device stage-2 mappings to Normal-NC attributes does not > change anything in this respect, in that the mismatched aliases would > only affect (refer to [2] for a detailed explanation) ordering between > the userspace and GuestOS mappings resulting stream of transactions > (i.e. it does not cause loss of property for either stream of > transactions on its own), which is harmless given that the userspace > and GuestOS access to the device is carried out through independent > transactions streams. > > [1] section D8.5 - DDI0487_I_a_a-profile_architecture_reference_manual.pdf > [2] section B2.8 - DDI0487_I_a_a-profile_architecture_reference_manual.pdf > [3] sections 1.7.7.3/1.8.5.2/appendix C - DEN0029H_SBSA_7.1.pdf > > Applied over next-20231201 > > History > ======= > v1 -> v2 > - Updated commit log to the one posted by > Lorenzo Pieralisi <lpieralisi@xxxxxxxxxx> (Thanks!) > - Added new flag to represent the NORMAL_NC setting. Updated > stage2_set_prot_attr() to handle new flag. > > v1 Link: > https://lore.kernel.org/all/20230907181459.18145-3-ankita@xxxxxxxxxx/ > > Signed-off-by: Ankit Agrawal <ankita@xxxxxxxxxx> > Suggested-by: Jason Gunthorpe <jgg@xxxxxxxxxx> > Acked-by: Catalin Marinas <catalin.marinas@xxxxxxx> In the light of having to keep this relaxation only for PCIe devices, I will withdraw my Ack until we come to a conclusion. Thanks. -- Catalin
