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[PATCH v2 13/18] target/riscv/kvm.c: update KVM MISA bits

From: Daniel Henrique Barboza
Subject: [PATCH v2 13/18] target/riscv/kvm.c: update KVM MISA bits
Date: Tue, 13 Jun 2023 17:58:52 -0300

Our design philosophy with KVM properties can be resumed in two main
decisions based on KVM interface availability and what the user wants to

- if the user disables an extension that the host KVM module doesn't
know about (i.e. it doesn't implement the kvm_get_one_reg() interface),
keep booting the CPU. This will avoid users having to deal with issues
with older KVM versions while disabling features they don't care;

- for any other case we're going to error out immediately. If the user
wants to enable a feature that KVM doesn't know about this a problem that
is worth aborting - the user must know that the feature wasn't enabled
in the hart. Likewise, if KVM knows about the extension, the user wants
to enable/disable it, and we fail to do it so, that's also a problem we
can't shrug it off.

For MISA bits we're going to be a little more conservative: we won't
even try enabling bits that aren't already available in the host. The
ioctl() is so likely to fail that's not worth trying. This check is
already done in the previous patch, in kvm_cpu_set_misa_ext_cfg(), thus
we don't need to worry about it now.

In kvm_riscv_update_cpu_misa_ext() we'll go through every potential user
option and do as follows:

- if the user didn't set the property or set to the same value of the
host, do nothing;

- Disable the given extension in KVM. Error out if anything goes wrong.

Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
 target/riscv/kvm.c | 40 ++++++++++++++++++++++++++++++++++++++++
 1 file changed, 40 insertions(+)

diff --git a/target/riscv/kvm.c b/target/riscv/kvm.c
index 53042a0e86..ea38f91b92 100644
--- a/target/riscv/kvm.c
+++ b/target/riscv/kvm.c
@@ -164,6 +164,41 @@ static void kvm_cpu_set_misa_ext_cfg(Object *obj, Visitor 
                "enabled in the host", misa_ext_cfg->name);
+static void kvm_riscv_update_cpu_misa_ext(RISCVCPU *cpu, CPUState *cs)
+    CPURISCVState *env = &cpu->env;
+    uint64_t id, reg;
+    int i, ret;
+    for (i = 0; i < ARRAY_SIZE(kvm_misa_ext_cfgs); i++) {
+        KVMCPUConfig *misa_cfg = &kvm_misa_ext_cfgs[i];
+        target_ulong misa_bit = misa_cfg->offset;
+        if (!misa_cfg->user_set) {
+            continue;
+        }
+        /* If we're here we're going to disable the MISA bit */
+        reg = 0;
+        id = kvm_riscv_reg_id(env, KVM_REG_RISCV_ISA_EXT,
+                              misa_cfg->kvm_reg_id);
+        ret = kvm_set_one_reg(cs, id, &reg);
+        if (ret != 0) {
+            /*
+             * We're not checking for -EINVAL because if the bit is
+             * about to be disabled means that it was already enabled
+             * by KVM, something that we determined by fetching the
+             * 'isa' register during init() time. Any error at this
+             * point is worth aborting.
+             */
+            error_report("Unable to set KVM reg %s, error %d",
+                         misa_cfg->name, ret);
+            exit(EXIT_FAILURE);
+        }
+        env->misa_ext &= ~misa_bit;
+    }
 static void kvm_riscv_add_cpu_user_properties(Object *cpu_obj)
     int i;
@@ -630,8 +665,13 @@ int kvm_arch_init_vcpu(CPUState *cs)
     if (!object_dynamic_cast(OBJECT(cpu), TYPE_RISCV_CPU_HOST)) {
         ret = kvm_vcpu_set_machine_ids(cpu, cs);
+        if (ret != 0) {
+            return ret;
+        }
+    kvm_riscv_update_cpu_misa_ext(cpu, cs);
     return ret;

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