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V" , Emi Kisanuki , Vishal Annapurve , WeiLin.Chang@arm.com, Lorenzo Pieralisi Subject: Re: [PATCH v16 44/45] KVM: arm64: CCA: Require ICH_HCR_EL2.TDIR for realms Message-ID: References: <20260803134403.80630-1-steven.price@arm.com> <20260803134403.80630-45-steven.price@arm.com> Precedence: bulk X-Mailing-List: kvmarm@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: On 08/24 15:50, Steven Price wrote: > On 10/08/2026 05:58, Kohei Enju wrote: > > On 08/03 14:44, Steven Price wrote: > >> KVM advertises realm support when the RMM is available, and allows > >> userspace to create a VM with KVM_VM_TYPE_ARM_REALM on that basis. > >> > >> On CPUs that lack ICH_HCR_EL2.TDIR, KVM uses ICH_HCR_EL2.TC for > >> normal guests so that ICC_DIR_EL1 is still trapped via the common GICv3 > >> CPU interface trap. Realms cannot rely on the normal hyp-side trap > >> handling for that fallback, so advertising RMI support on such systems > >> lets userspace create a realm that cannot safely run. > >> > >> Require the finalized ARM64_HAS_ICH_HCR_EL2_TDIR capability when > >> reporting KVM_CAP_ARM_RMI and when accepting KVM_VM_TYPE_ARM_REALM. > >> This leaves normal VM creation unchanged on systems that need the TC > >> workaround. > > > > Hi Steven, > > Hi Kohei, > > Sorry for the slow response. > > > Thanks for your work on upstreaming CCA. > > > > In the v15 discussion [0], you asked whether the system I was testing was a > > "hacked up test system" or closer to "production hardware", and I said I would > > share more when the time came. I can now say that this is not a hacked-up test > > system. At Fujitsu, we have real hardware (FUJITSU-MONAKA) which implements CCA > > (FEAT_RME) but does not implement FEAT_GICv3_TDIR. The hardware details are as > > follows: > > Cool, I suspected that might be the case - it's good to know there's > real hardware on it's way. > > > - GICv4.2 compliant implementation > > - Supports FEAT_GICv3, FEAT_GICv3p1, FEAT_GICv4, FEAT_GICv4p1, and FEAT_GICv3_NMI > > - Does not support FEAT_GICv3_LEGACY (deprecated) > > - Does not support FEAT_GICv3_TDIR (ICH_VTR_EL2.TDS == 0) > > > > For reference, compared with Arm Neoverse V3, the virtual GIC configuration is > > largely equivalent. The only missing non-deprecated architectural feature is > > FEAT_GICv3_TDIR. > > > > The issue I see is that the CCA KVM code currently does not support a > > configuration (non-TDIR/common-trap) that normal KVM already supports. For > > normal guests, KVM handles systems without TDIR by using ICH_HCR_EL2.TC and the > > existing GICv3 CPU interface emulation path. However, Realm guests currently > > fail because the CCA path bypasses that existing emulation path, as Marc also > > pointed out in [1]. > > > > Also, this is not limited to systems that actually lack TDIR. The same failure > > can be reproduced on a TDIR-capable system by booting with: > > kvm-arm.vgic_v3_common_trap=1 > > As Marc says that's a debugging option - handy for those of us who don't > have a platform without TDIR to test with. > > > So it seems that the current CCA KVM implementation does not yet cover a > > configuration that normal KVM already supports today, rather than this being a > > limitation of the RMM specification or the underlying hardware. > > > > I've included a patch below which reuses the existing GICv3 early emulation > > path for Realm sysreg exits. This patch does not add any new vGIC emulation > > code, and leaves the existing vGIC emulation code unchanged. So I believe this > > is in line with Marc's request in [1]. With this patch, Realm guests can run > > when the common CPU interface trap path is enabled. > > > > I tested the exact patch both on our real silicon and on QEMU, and > > confirmed that all Realm-related tests in kvm-unit-tests-cca passed. > > > > I'm not attached to this exact implementation, and I'm happy if the solution is > > reworked to better fit into the next revision. > > > > Given that this configuration can be supported by reusing the existing KVM > > emulation infrastructure, I think it would be reasonable for CCA to support the > > non-TDIR/common-trap configuration rather than requiring ICH_HCR_EL2.TDIR > > unconditionally for Realm support. > > > > Supporting this configuration would also allow us to validate the upstream CCA > > KVM implementation on real silicon using upstream code paths, and contribute > > additional real-hardware testing coverage as the implementation > > evolves. > > > > I'd be very interested in hearing your thoughts. > > So personally I think your patch is a good compromise. It gets the > hardware working and I'm keen to enable real-hardware testing. Marc has > a very valid point that in terms of performance this could be very bad. > Pseudo NMI in particular will be terrible because accesses to GIC > registers are used to "emulate" the NMI so the number of traps will be > large, and the traps are much more expensive with CCA. Yes, in that case ICV_PMR_EL1 would be accessed frequently, and the resulting traps would be expensive. > > So I'll attempt to incorporate the changes in your patch, but obviously > you'll have to decide for yourself whether the performance of the > product is suitable. Thanks for the clarification. I agree with the performance concern, and I'll run some benchmarks on our hardware. Thanks, Kohei > > Thanks, > Steve >