Re: [PATCH 06/16] target/riscv: use KVM scratch CPUs to init KVM properties

[Alistair Francis]

On Wed, May 31, 2023 at 5:50 AM Daniel Henrique Barboza
<dbarboza@ventanamicro.com> wrote:
>
> Certain validations, such as the validations done for the machine IDs
> (mvendorid/marchid/mimpid), are done before starting the CPU.
> Non-dynamic (named) CPUs tries to match user input with a preset
> default. As it is today we can't prefetch a KVM default for these cases
> because we're only able to read/write KVM regs after the vcpu is
> spinning.
>
> Our target/arm friends use a concept called "scratch CPU", which
> consists of creating a vcpu for doing queries and validations and so on,
> which is discarded shortly after use [1]. This is a suitable solution
> for what we need so let's implement it in target/riscv as well.
>
> kvm_riscv_init_machine_ids() will be used to do any pre-launch setup for
> KVM CPUs, via riscv_cpu_add_user_properties(). The function will create
> a KVM scratch CPU, fetch KVM regs that work as default values for user
> properties, and then discard the scratch CPU afterwards.
>
> We're starting by initializing 'mvendorid'. This concept will be used to
> init other KVM specific properties in the next patches as well.
>
> [1] target/arm/kvm.c, kvm_arm_create_scratch_host_vcpu()
>
> Suggested-by: Andrew Jones <ajones@ventanamicro.com>
> Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>

Acked-by: Alistair Francis <alistair.francis@wdc.com>

Alistair

> ---
>  target/riscv/cpu.c       |  4 ++
>  target/riscv/kvm.c       | 85 ++++++++++++++++++++++++++++++++++++++++
>  target/riscv/kvm_riscv.h |  1 +
>  3 files changed, 90 insertions(+)
>
> diff --git a/target/riscv/cpu.c b/target/riscv/cpu.c
> index d6e23bfd83..749d8bf5eb 100644
> --- a/target/riscv/cpu.c
> +++ b/target/riscv/cpu.c
> @@ -1703,6 +1703,10 @@ static void riscv_cpu_add_user_properties(Object *obj)
>      Property *prop;
>      DeviceState *dev = DEVICE(obj);
>
> +    if (riscv_running_KVM()) {
> +        kvm_riscv_init_user_properties(obj);
> +    }
> +
>      riscv_cpu_add_misa_properties(obj);
>
>      for (prop = riscv_cpu_extensions; prop && prop->name; prop++) {
> diff --git a/target/riscv/kvm.c b/target/riscv/kvm.c
> index 0f932a5b96..37f0f70794 100644
> --- a/target/riscv/kvm.c
> +++ b/target/riscv/kvm.c
> @@ -309,6 +309,91 @@ static void kvm_riscv_put_regs_timer(CPUState *cs)
>      env->kvm_timer_dirty = false;
>  }
>
> +typedef struct KVMScratchCPU {
> +    int kvmfd;
> +    int vmfd;
> +    int cpufd;
> +} KVMScratchCPU;
> +
> +/*
> + * Heavily inspired by kvm_arm_create_scratch_host_vcpu()
> + * from target/arm/kvm.c.
> + */
> +static bool kvm_riscv_create_scratch_vcpu(KVMScratchCPU *scratch)
> +{
> +    int kvmfd = -1, vmfd = -1, cpufd = -1;
> +
> +    kvmfd = qemu_open_old("/dev/kvm", O_RDWR);
> +    if (kvmfd < 0) {
> +        goto err;
> +    }
> +    do {
> +        vmfd = ioctl(kvmfd, KVM_CREATE_VM, 0);
> +    } while (vmfd == -1 && errno == EINTR);
> +    if (vmfd < 0) {
> +        goto err;
> +    }
> +    cpufd = ioctl(vmfd, KVM_CREATE_VCPU, 0);
> +    if (cpufd < 0) {
> +        goto err;
> +    }
> +
> +    scratch->kvmfd =  kvmfd;
> +    scratch->vmfd = vmfd;
> +    scratch->cpufd = cpufd;
> +
> +    return true;
> +
> + err:
> +    if (cpufd >= 0) {
> +        close(cpufd);
> +    }
> +    if (vmfd >= 0) {
> +        close(vmfd);
> +    }
> +    if (kvmfd >= 0) {
> +        close(kvmfd);
> +    }
> +
> +    return false;
> +}
> +
> +static void kvm_riscv_destroy_scratch_vcpu(KVMScratchCPU *scratch)
> +{
> +    close(scratch->cpufd);
> +    close(scratch->vmfd);
> +    close(scratch->kvmfd);
> +}
> +
> +static void kvm_riscv_init_machine_ids(RISCVCPU *cpu, KVMScratchCPU *kvmcpu)
> +{
> +    CPURISCVState *env = &cpu->env;
> +    struct kvm_one_reg reg;
> +    int ret;
> +
> +    reg.id = kvm_riscv_reg_id(env, KVM_REG_RISCV_CONFIG,
> +                              KVM_REG_RISCV_CONFIG_REG(mvendorid));
> +    reg.addr = (uint64_t)&cpu->cfg.mvendorid;
> +    ret = ioctl(kvmcpu->cpufd, KVM_GET_ONE_REG, &reg);
> +    if (ret != 0) {
> +        error_report("Unable to retrieve mvendorid from host, error %d", 
> ret);
> +    }
> +}
> +
> +void kvm_riscv_init_user_properties(Object *cpu_obj)
> +{
> +    RISCVCPU *cpu = RISCV_CPU(cpu_obj);
> +    KVMScratchCPU kvmcpu;
> +
> +    if (!kvm_riscv_create_scratch_vcpu(&kvmcpu)) {
> +        return;
> +    }
> +
> +    kvm_riscv_init_machine_ids(cpu, &kvmcpu);
> +
> +    kvm_riscv_destroy_scratch_vcpu(&kvmcpu);
> +}
> +
>  const KVMCapabilityInfo kvm_arch_required_capabilities[] = {
>      KVM_CAP_LAST_INFO
>  };
> diff --git a/target/riscv/kvm_riscv.h b/target/riscv/kvm_riscv.h
> index ed281bdce0..e3ba935808 100644
> --- a/target/riscv/kvm_riscv.h
> +++ b/target/riscv/kvm_riscv.h
> @@ -19,6 +19,7 @@
>  #ifndef QEMU_KVM_RISCV_H
>  #define QEMU_KVM_RISCV_H
>
> +void kvm_riscv_init_user_properties(Object *cpu_obj);
>  void kvm_riscv_reset_vcpu(RISCVCPU *cpu);
>  void kvm_riscv_set_irq(RISCVCPU *cpu, int irq, int level);
>
> --
> 2.40.1
>
>