Hi Marc,

-----Original Message----- From: Marc Zyngier [mailto:maz@kernel.org] Sent: 08 January 2021 17:12 To: linux-arm-kernel@lists.infradead.org; kvmarm@lists.cs.columbia.edu Cc: Shameerali Kolothum Thodi shameerali.kolothum.thodi@huawei.com; James Morse james.morse@arm.com; Julien Thierry julien.thierry.kdev@gmail.com; Suzuki K Poulose suzuki.poulose@arm.com; Ard Biesheuvel ardb@kernel.org; kernel-team@android.com Subject: [PATCH 2/2] KVM: arm64: Workaround firmware wrongly advertising GICv2-on-v3 compatibility

It looks like we have broken firmware out there that wrongly advertises a GICv2 compatibility interface, despite the CPUs not being able to deal with it.

To work around this, check that the CPU initialising KVM is actually able to switch to MMIO instead of system registers, and use that as a precondition to enable GICv2 compatibility in KVM.

Note that the detection happens on a single CPU. If the firmware is lying *and* that the CPUs are asymetric, all hope is lost anyway.

Reported-by: Shameerali Kolothum Thodi shameerali.kolothum.thodi@huawei.com Signed-off-by: Marc Zyngier maz@kernel.org

---

arch/arm64/kvm/hyp/vgic-v3-sr.c | 34 +++++++++++++++++++++++++++++++-- arch/arm64/kvm/vgic/vgic-v3.c | 8 ++++++-- 2 files changed, 38 insertions(+), 4 deletions(-)

diff --git a/arch/arm64/kvm/hyp/vgic-v3-sr.c b/arch/arm64/kvm/hyp/vgic-v3-sr.c index 005daa0c9dd7..d504499ab917 100644 --- a/arch/arm64/kvm/hyp/vgic-v3-sr.c +++ b/arch/arm64/kvm/hyp/vgic-v3-sr.c @@ -408,11 +408,41 @@ void __vgic_v3_init_lrs(void) /*

• Return the GIC CPU configuration:
• • [31:0] ICH_VTR_EL2

• • • [63:32] RES0

• • • [62:32] RES0
• • • [63] MMIO (GICv2) capable
  */

u64 __vgic_v3_get_gic_config(void) {

• return read_gicreg(ICH_VTR_EL2);

• u64 sre = read_gicreg(ICC_SRE_EL1);
• unsigned long flags = 0;
• bool v2_capable;
• /*

• * To check whether we have a MMIO-based (GICv2 compatible)
  

• * CPU interface, we need to disable the system register
  

• * view. To do that safely, we have to prevent any interrupt
  

• * from firing (which would be deadly).
  

• *
  

• * Note that this only makes sense on VHE, as interrupts are
  

• * already masked for nVHE as part of the exception entry to
  

• * EL2.
  

• */
  

• if (has_vhe())

• flags = local_daif_save();
  

• write_gicreg(0, ICC_SRE_EL1);
• isb();
• v2_capable = !(read_gicreg(ICC_SRE_EL1) & ICC_SRE_EL1_SRE);
• write_gicreg(sre, ICC_SRE_EL1);
• isb();
• if (has_vhe())

• local_daif_restore(flags);
  

• return (read_gicreg(ICH_VTR_EL2) |

• v2_capable ? (1ULL << 63) : 0);
  

}

Thanks for sending this out. I had a go with this series and unfortunately it didn't work on a system with faulty BIOS. It looks like the culprit here is the ?: operator. There seems to be an operator precedence at play here and it returns, vgic_v3_probe: ich_vtr_el2 0x8000000000000000

And with the below change,

return (read_gicreg(ICH_VTR_EL2) | - v2_capable ? (1ULL << 63) : 0); + (v2_capable ? (1ULL << 63) : 0)); }

It returns, vgic_v3_probe: ich_vtr_el2 0x90080003

and works correctly. [ 18.918738] kvm [1]: disabling GICv2 emulation

Thanks, Shameer

u64 __vgic_v3_read_vmcr(void) diff --git a/arch/arm64/kvm/vgic/vgic-v3.c b/arch/arm64/kvm/vgic/vgic-v3.c index 8e7bf3151057..67b27b47312b 100644 --- a/arch/arm64/kvm/vgic/vgic-v3.c +++ b/arch/arm64/kvm/vgic/vgic-v3.c @@ -584,8 +584,10 @@ early_param("kvm-arm.vgic_v4_enable", early_gicv4_enable); int vgic_v3_probe(const struct gic_kvm_info *info) { u64 ich_vtr_el2 = kvm_call_hyp_ret(__vgic_v3_get_gic_config);

• bool has_v2; int ret;

• has_v2 = ich_vtr_el2 >> 63; ich_vtr_el2 = (u32)ich_vtr_el2;

  /*

@@ -605,13 +607,15 @@ int vgic_v3_probe(const struct gic_kvm_info *info) gicv4_enable ? "en" : "dis"); }

• kvm_vgic_global_state.vcpu_base = 0;
• if (!info->vcpu.start) { kvm_info("GICv3: no GICV resource entry\n");

• kvm_vgic_global_state.vcpu_base = 0;
  

• } else if (!has_v2) {

• pr_warn("CPU interface incapable of MMIO access\n");
  

  } else if (!PAGE_ALIGNED(info->vcpu.start)) { pr_warn("GICV physical address 0x%llx not page aligned\n", (unsigned long long)info->vcpu.start);

• kvm_vgic_global_state.vcpu_base = 0;
  

  } else { kvm_vgic_global_state.vcpu_base = info->vcpu.start; kvm_vgic_global_state.can_emulate_gicv2 = true;

-- 2.29.2