On Tue, Feb 20, 2024 at 12:59:23PM +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. > > The NormalNC 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. > > A Normal-NC flag is not present today. So add a new kvm_pgtable_prot > (KVM_PGTABLE_PROT_NORMAL_NC) flag for it, along with its > corresponding PTE value 0x5 (0b101) determined from [1]. > > Lastly, adapt the stage2 PTE property setter function > (stage2_set_prot_attr) to handle the NormalNC attribute. > > [1] section D8.5.5 - DDI0487J_a_a-profile_architecture_reference_manual.pdf > [2] section B2.8 - DDI0487J_a_a-profile_architecture_reference_manual.pdf > [3] sections 1.7.7.3/1.8.5.2/appendix C - DEN0029H_SBSA_7.1.pdf > A couple of Link: tags would not hurt in my opinion, lest we forget: Link: https://lore.kernel.org/r/20230907181459.18145-2-ankita@xxxxxxxxxx/ Link: https://lore.kernel.org/r/20231205033015.10044-1-ankita@xxxxxxxxxx > Suggested-by: Jason Gunthorpe <jgg@xxxxxxxxxx> > Acked-by: Catalin Marinas <catalin.marinas@xxxxxxx> > Signed-off-by: Ankit Agrawal <ankita@xxxxxxxxxx> > --- > arch/arm64/include/asm/kvm_pgtable.h | 2 ++ > arch/arm64/include/asm/memory.h | 2 ++ > arch/arm64/kvm/hyp/pgtable.c | 24 +++++++++++++++++++----- > 3 files changed, 23 insertions(+), 5 deletions(-) > > diff --git a/arch/arm64/include/asm/kvm_pgtable.h b/arch/arm64/include/asm/kvm_pgtable.h > index cfdf40f734b1..19278dfe7978 100644 > --- a/arch/arm64/include/asm/kvm_pgtable.h > +++ b/arch/arm64/include/asm/kvm_pgtable.h > @@ -197,6 +197,7 @@ enum kvm_pgtable_stage2_flags { > * @KVM_PGTABLE_PROT_W: Write permission. > * @KVM_PGTABLE_PROT_R: Read permission. > * @KVM_PGTABLE_PROT_DEVICE: Device attributes. > + * @KVM_PGTABLE_PROT_NORMAL_NC: Normal noncacheable attributes. > * @KVM_PGTABLE_PROT_SW0: Software bit 0. > * @KVM_PGTABLE_PROT_SW1: Software bit 1. > * @KVM_PGTABLE_PROT_SW2: Software bit 2. > @@ -208,6 +209,7 @@ enum kvm_pgtable_prot { > KVM_PGTABLE_PROT_R = BIT(2), > > KVM_PGTABLE_PROT_DEVICE = BIT(3), > + KVM_PGTABLE_PROT_NORMAL_NC = BIT(4), > > KVM_PGTABLE_PROT_SW0 = BIT(55), > KVM_PGTABLE_PROT_SW1 = BIT(56), > diff --git a/arch/arm64/include/asm/memory.h b/arch/arm64/include/asm/memory.h > index d82305ab420f..449ca2ff1df6 100644 > --- a/arch/arm64/include/asm/memory.h > +++ b/arch/arm64/include/asm/memory.h > @@ -173,6 +173,7 @@ > * Memory types for Stage-2 translation > */ > #define MT_S2_NORMAL 0xf > +#define MT_S2_NORMAL_NC 0x5 > #define MT_S2_DEVICE_nGnRE 0x1 > > /* > @@ -180,6 +181,7 @@ > * Stage-2 enforces Normal-WB and Device-nGnRE > */ > #define MT_S2_FWB_NORMAL 6 > +#define MT_S2_FWB_NORMAL_NC 5 > #define MT_S2_FWB_DEVICE_nGnRE 1 > > #ifdef CONFIG_ARM64_4K_PAGES > diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c > index ab9d05fcf98b..3fae5830f8d2 100644 > --- a/arch/arm64/kvm/hyp/pgtable.c > +++ b/arch/arm64/kvm/hyp/pgtable.c > @@ -717,15 +717,29 @@ void kvm_tlb_flush_vmid_range(struct kvm_s2_mmu *mmu, > static int stage2_set_prot_attr(struct kvm_pgtable *pgt, enum kvm_pgtable_prot prot, > kvm_pte_t *ptep) > { > - bool device = prot & KVM_PGTABLE_PROT_DEVICE; > - kvm_pte_t attr = device ? KVM_S2_MEMATTR(pgt, DEVICE_nGnRE) : > - KVM_S2_MEMATTR(pgt, NORMAL); > + kvm_pte_t attr; > u32 sh = KVM_PTE_LEAF_ATTR_LO_S2_SH_IS; > > + switch (prot & (KVM_PGTABLE_PROT_DEVICE | > + KVM_PGTABLE_PROT_NORMAL_NC)) { > + case KVM_PGTABLE_PROT_DEVICE | KVM_PGTABLE_PROT_NORMAL_NC: > + return -EINVAL; > + case KVM_PGTABLE_PROT_DEVICE: > + if (prot & KVM_PGTABLE_PROT_X) > + return -EINVAL; > + attr = KVM_S2_MEMATTR(pgt, DEVICE_nGnRE); > + break; > + case KVM_PGTABLE_PROT_NORMAL_NC: > + if (prot & KVM_PGTABLE_PROT_X) > + return -EINVAL; > + attr = KVM_S2_MEMATTR(pgt, NORMAL_NC); > + break; > + default: > + attr = KVM_S2_MEMATTR(pgt, NORMAL); > + } > + > if (!(prot & KVM_PGTABLE_PROT_X)) > attr |= KVM_PTE_LEAF_ATTR_HI_S2_XN; > - else if (device) > - return -EINVAL; > > if (prot & KVM_PGTABLE_PROT_R) > attr |= KVM_PTE_LEAF_ATTR_LO_S2_S2AP_R; > -- > 2.34.1 >
