[v3] target/riscv/kvm: skip FP/Vector sync on KVM_PUT_RUNTIME_STATE

[PATCH v3] target/riscv/kvm: skip FP/Vector sync on KVM_PUT_RUNTIME_STATE

Posted by Meng Zhuo 1 week, 5 days ago

During KVM exit processing (KVM_PUT_RUNTIME_STATE), QEMU never modifies
FP or Vector registers — only core GPRs/PC and CSRs are potentially
changed.  Re-syncing 32 FP and 32 Vector registers on every KVM exit
wastes 36-68 individual KVM_SET_ONE_REG ioctls per vCPU exit.

Follow the s390x pattern: early return on RUNTIME after syncing core
registers and CSRs.  KVM_PUT_FULL_STATE and KVM_PUT_RESET_STATE continue
to sync everything for correctness during migration and initialization.

Signed-off-by: Meng Zhuo <mengzhuo@iscas.ac.cn>
---
 target/riscv/kvm/kvm-cpu.c | 19 ++++++++++++++-----
 1 file changed, 14 insertions(+), 5 deletions(-)

diff --git a/target/riscv/kvm/kvm-cpu.c b/target/riscv/kvm/kvm-cpu.c
index b047ffa9c0..dd4193ea8b 100644
--- a/target/riscv/kvm/kvm-cpu.c
+++ b/target/riscv/kvm/kvm-cpu.c
@@ -1385,6 +1385,17 @@ int kvm_arch_put_registers(CPUState *cs, KvmPutState level, Error **errp)
         return ret;
     }
 
+    /*
+     * For RUNTIME_STATE, KVM already has the correct FP and Vector state
+     * from the preceding KVM_RUN exit. QEMU never modifies these registers
+     * during exit handling, so re-syncing is unnecessary.  This saves ~68
+     * KVM_SET_ONE_REG ioctls per vCPU exit.  See also s390x which uses
+     * the same pattern.
+     */
+    if (KVM_PUT_RUNTIME_STATE == level) {
+        return ret;
+    }
+
     ret = kvm_riscv_put_regs_fp(cs);
     if (ret) {
         return ret;
@@ -1397,11 +1408,9 @@ int kvm_arch_put_registers(CPUState *cs, KvmPutState level, Error **errp)
 
     if (KVM_PUT_RESET_STATE == level) {
         RISCVCPU *cpu = RISCV_CPU(cs);
-        if (cs->cpu_index == 0) {
-            ret = kvm_riscv_sync_mpstate_to_kvm(cpu, KVM_MP_STATE_RUNNABLE);
-        } else {
-            ret = kvm_riscv_sync_mpstate_to_kvm(cpu, KVM_MP_STATE_STOPPED);
-        }
+        int state = cs->cpu_index == 0 ? KVM_MP_STATE_RUNNABLE
+                                       : KVM_MP_STATE_STOPPED;
+        ret = kvm_riscv_sync_mpstate_to_kvm(cpu, state);
         if (ret) {
             return ret;
         }
-- 
2.47.3

Re: [PATCH v3] target/riscv/kvm: skip FP/Vector sync on KVM_PUT_RUNTIME_STATE

Posted by Chao Liu 1 week, 4 days ago

On Mon, May 18, 2026 at 06:21:18PM +0800, Meng Zhuo wrote:
> During KVM exit processing (KVM_PUT_RUNTIME_STATE), QEMU never modifies
> FP or Vector registers — only core GPRs/PC and CSRs are potentially
> changed.  Re-syncing 32 FP and 32 Vector registers on every KVM exit
> wastes 36-68 individual KVM_SET_ONE_REG ioctls per vCPU exit.
> 
> Follow the s390x pattern: early return on RUNTIME after syncing core
> registers and CSRs.  KVM_PUT_FULL_STATE and KVM_PUT_RESET_STATE continue
> to sync everything for correctness during migration and initialization.
> 
> Signed-off-by: Meng Zhuo <mengzhuo@iscas.ac.cn>
Reviewed-by: Chao Liu <chao.liu.zevorn@gmail.com>

Thanks,
Chao
> ---
>  target/riscv/kvm/kvm-cpu.c | 19 ++++++++++++++-----
>  1 file changed, 14 insertions(+), 5 deletions(-)
> 
> diff --git a/target/riscv/kvm/kvm-cpu.c b/target/riscv/kvm/kvm-cpu.c
> index b047ffa9c0..dd4193ea8b 100644
> --- a/target/riscv/kvm/kvm-cpu.c
> +++ b/target/riscv/kvm/kvm-cpu.c
> @@ -1385,6 +1385,17 @@ int kvm_arch_put_registers(CPUState *cs, KvmPutState level, Error **errp)
>          return ret;
>      }
>  
> +    /*
> +     * For RUNTIME_STATE, KVM already has the correct FP and Vector state
> +     * from the preceding KVM_RUN exit. QEMU never modifies these registers
> +     * during exit handling, so re-syncing is unnecessary.  This saves ~68
> +     * KVM_SET_ONE_REG ioctls per vCPU exit.  See also s390x which uses
> +     * the same pattern.
> +     */
> +    if (KVM_PUT_RUNTIME_STATE == level) {
> +        return ret;
> +    }
> +
>      ret = kvm_riscv_put_regs_fp(cs);
>      if (ret) {
>          return ret;
> @@ -1397,11 +1408,9 @@ int kvm_arch_put_registers(CPUState *cs, KvmPutState level, Error **errp)
>  
>      if (KVM_PUT_RESET_STATE == level) {
>          RISCVCPU *cpu = RISCV_CPU(cs);
> -        if (cs->cpu_index == 0) {
> -            ret = kvm_riscv_sync_mpstate_to_kvm(cpu, KVM_MP_STATE_RUNNABLE);
> -        } else {
> -            ret = kvm_riscv_sync_mpstate_to_kvm(cpu, KVM_MP_STATE_STOPPED);
> -        }
> +        int state = cs->cpu_index == 0 ? KVM_MP_STATE_RUNNABLE
> +                                       : KVM_MP_STATE_STOPPED;
> +        ret = kvm_riscv_sync_mpstate_to_kvm(cpu, state);
>          if (ret) {
>              return ret;
>          }
> -- 
> 2.47.3
> 
>