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Re: [PATCH 2/3] target/arm: Set KVM_ARM_VCPU_SVE while probing the host


From: Richard Henderson
Subject: Re: [PATCH 2/3] target/arm: Set KVM_ARM_VCPU_SVE while probing the host
Date: Mon, 25 Jul 2022 11:20:23 -0700
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.11.0

On 7/25/22 11:14, Richard Henderson wrote:
Because we weren't setting this flag, our probe of ID_AA64ZFR0
was always returning zero.  This also obviates the adjustment
of ID_AA64PFR0, which had sanitized the SVE field.


Oh, I meant to say here that this the effects of the bug are not visible, because the only thing that ISAR.ID_AA64ZFR0 is used for within qemu at present is tcg translation. The other tests for SVE within KVM are via ISAR.ID_AA64PFR0.SVE.


r~



Reported-by: Zenghui Yu <yuzenghui@huawei.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
---
  target/arm/kvm64.c | 27 +++++++++++++--------------
  1 file changed, 13 insertions(+), 14 deletions(-)

diff --git a/target/arm/kvm64.c b/target/arm/kvm64.c
index 36ff20c9e3..8b2ae9948a 100644
--- a/target/arm/kvm64.c
+++ b/target/arm/kvm64.c
@@ -512,7 +512,6 @@ bool kvm_arm_get_host_cpu_features(ARMHostCPUFeatures *ahcf)
      bool sve_supported;
      bool pmu_supported = false;
      uint64_t features = 0;
-    uint64_t t;
      int err;
/* Old kernels may not know about the PREFERRED_TARGET ioctl: however
@@ -533,10 +532,17 @@ bool kvm_arm_get_host_cpu_features(ARMHostCPUFeatures 
*ahcf)
      struct kvm_vcpu_init init = { .target = -1, };
/*
-     * Ask for Pointer Authentication if supported. We can't play the
-     * SVE trick of synthesising the ID reg as KVM won't tell us
-     * whether we have the architected or IMPDEF version of PAuth, so
-     * we have to use the actual ID regs.
+     * Ask for SVE if supported, so that we can query ID_AA64ZFR0,
+     * which is otherwise RAZ.
+     */
+    sve_supported = kvm_arm_svm_supported();
+    if (sve_supported) {
+        init.features[0] |= 1 << KVM_ARM_VCPU_SVE;
+    }
+
+    /*
+     * Ask for Pointer Authentication if supported, so that we get
+     * the unsanitized field values for AA64ISAR1_EL1.
       */
      if (kvm_arm_pauth_supported()) {
          init.features[0] |= (1 << KVM_ARM_VCPU_PTRAUTH_ADDRESS |
@@ -680,20 +686,13 @@ bool kvm_arm_get_host_cpu_features(ARMHostCPUFeatures 
*ahcf)
          }
      }
- sve_supported = kvm_arm_svm_supported();
-
-    /* Add feature bits that can't appear until after VCPU init. */
      if (sve_supported) {
-        t = ahcf->isar.id_aa64pfr0;
-        t = FIELD_DP64(t, ID_AA64PFR0, SVE, 1);
-        ahcf->isar.id_aa64pfr0 = t;
-
          /*
           * There is a range of kernels between kernel commit 73433762fcae
           * and f81cb2c3ad41 which have a bug where the kernel doesn't expose
           * SYS_ID_AA64ZFR0_EL1 via the ONE_REG API unless the VM has enabled
-         * SVE support, so we only read it here, rather than together with all
-         * the other ID registers earlier.
+         * SVE support, which resulted in an error rather than RAZ.
+         * So only read the register if we set KVM_ARM_VCPU_SVE above.
           */
          err |= read_sys_reg64(fdarray[2], &ahcf->isar.id_aa64zfr0,
                                ARM64_SYS_REG(3, 0, 0, 4, 4));




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