target/loongarch/kvm: set pv features per vCPU in kvm_arch_init_vcpu

[PATCH] target/loongarch/kvm: set pv features per vCPU in kvm_arch_init_vcpu

Posted by Tao Cui 1 month ago

From: Tao Cui <cuitao@kylinos.cn>

kvm_set_pv_features() programs the KVM_FEATURE cpucfg attribute, which is a
per-vCPU setting. It was called from kvm_arch_put_registers() under a
function-local static guard, so it ran only once for the whole VM and only
the first vCPU actually got its pv features pushed to KVM.

Move the call to kvm_arch_init_vcpu(), which runs once per vCPU, right
after the pv features are computed, and drop the static guard.

Signed-off-by: Tao Cui <cuitao@kylinos.cn>
---
 target/loongarch/kvm/kvm.c | 15 ++++++---------
 1 file changed, 6 insertions(+), 9 deletions(-)

diff --git a/target/loongarch/kvm/kvm.c b/target/loongarch/kvm/kvm.c
index d6539c12ac..985812c828 100644
--- a/target/loongarch/kvm/kvm.c
+++ b/target/loongarch/kvm/kvm.c
@@ -816,7 +816,6 @@ int kvm_arch_get_registers(CPUState *cs, Error **errp)
 int kvm_arch_put_registers(CPUState *cs, KvmPutState level, Error **errp)
 {
     int ret;
-    static int once;
 
     ret = kvm_loongarch_put_regs_core(cs);
     if (ret) {
@@ -843,14 +842,6 @@ int kvm_arch_put_registers(CPUState *cs, KvmPutState level, Error **errp)
         return ret;
     }
 
-    if (!once) {
-        ret = kvm_set_pv_features(cs);
-        if (ret) {
-            return ret;
-        }
-        once = 1;
-    }
-
     if (level >= KVM_PUT_FULL_STATE) {
         /*
          * only KVM_PUT_FULL_STATE is required, kvm kernel will clear
@@ -1209,6 +1200,12 @@ int kvm_arch_init_vcpu(CPUState *cs)
         return ret;
     }
 
+    /* pv_features is a per-vCPU attribute; set it here, once per vCPU. */
+    ret = kvm_set_pv_features(cs);
+    if (ret < 0) {
+        return ret;
+    }
+
     ret = kvm_cpu_check_ptw(cs, &local_err);
     if (ret < 0) {
         error_report_err(local_err);
-- 
2.43.0

Re: [PATCH] target/loongarch/kvm: set pv features per vCPU in kvm_arch_init_vcpu

Posted by Bibo Mao 1 month ago


On 2026/6/25 上午9:53, Tao Cui wrote:
> From: Tao Cui <cuitao@kylinos.cn>
> 
> kvm_set_pv_features() programs the KVM_FEATURE cpucfg attribute, which is a
> per-vCPU setting. It was called from kvm_arch_put_registers() under a
> function-local static guard, so it ran only once for the whole VM and only
> the first vCPU actually got its pv features pushed to KVM.
> 
> Move the call to kvm_arch_init_vcpu(), which runs once per vCPU, right
> after the pv features are computed, and drop the static guard.
> 
> Signed-off-by: Tao Cui <cuitao@kylinos.cn>
> ---
>   target/loongarch/kvm/kvm.c | 15 ++++++---------
>   1 file changed, 6 insertions(+), 9 deletions(-)
> 
> diff --git a/target/loongarch/kvm/kvm.c b/target/loongarch/kvm/kvm.c
> index d6539c12ac..985812c828 100644
> --- a/target/loongarch/kvm/kvm.c
> +++ b/target/loongarch/kvm/kvm.c
> @@ -816,7 +816,6 @@ int kvm_arch_get_registers(CPUState *cs, Error **errp)
>   int kvm_arch_put_registers(CPUState *cs, KvmPutState level, Error **errp)
>   {
>       int ret;
> -    static int once;
>   
>       ret = kvm_loongarch_put_regs_core(cs);
>       if (ret) {
> @@ -843,14 +842,6 @@ int kvm_arch_put_registers(CPUState *cs, KvmPutState level, Error **errp)
>           return ret;
>       }
>   
> -    if (!once) {
> -        ret = kvm_set_pv_features(cs);
> -        if (ret) {
> -            return ret;
> -        }
> -        once = 1;
> -    }
> -
>       if (level >= KVM_PUT_FULL_STATE) {
>           /*
>            * only KVM_PUT_FULL_STATE is required, kvm kernel will clear
> @@ -1209,6 +1200,12 @@ int kvm_arch_init_vcpu(CPUState *cs)
>           return ret;
>       }
>   
> +    /* pv_features is a per-vCPU attribute; set it here, once per vCPU. */
> +    ret = kvm_set_pv_features(cs);
> +    if (ret < 0) {
> +        return ret;
> +    }
I prefer the old method. The pv_features is one vCPU state instead, and 
it will be added in vCPU VMState if migration is supported on different 
host kernel version.

Here is host feature detection, not vCPU state setting.

Regards
Bibo Mao
> +
>       ret = kvm_cpu_check_ptw(cs, &local_err);
>       if (ret < 0) {
>           error_report_err(local_err);
>

Re: [PATCH] target/loongarch/kvm: set pv features per vCPU in kvm_arch_init_vcpu

Posted by Tao Cui 3 weeks, 2 days ago


在 2026/6/25 11:14, Bibo Mao 写道:
> 
> 
> On 2026/6/25 上午9:53, Tao Cui wrote:
>> From: Tao Cui <cuitao@kylinos.cn>
>>
>> kvm_set_pv_features() programs the KVM_FEATURE cpucfg attribute, which is a
>> per-vCPU setting. It was called from kvm_arch_put_registers() under a
>> function-local static guard, so it ran only once for the whole VM and only
>> the first vCPU actually got its pv features pushed to KVM.
>>
>> Move the call to kvm_arch_init_vcpu(), which runs once per vCPU, right
>> after the pv features are computed, and drop the static guard.
>>
>> Signed-off-by: Tao Cui <cuitao@kylinos.cn>
>> ---
>>   target/loongarch/kvm/kvm.c | 15 ++++++---------
>>   1 file changed, 6 insertions(+), 9 deletions(-)
>>
>> diff --git a/target/loongarch/kvm/kvm.c b/target/loongarch/kvm/kvm.c
>> index d6539c12ac..985812c828 100644
>> --- a/target/loongarch/kvm/kvm.c
>> +++ b/target/loongarch/kvm/kvm.c
>> @@ -816,7 +816,6 @@ int kvm_arch_get_registers(CPUState *cs, Error **errp)
>>   int kvm_arch_put_registers(CPUState *cs, KvmPutState level, Error **errp)
>>   {
>>       int ret;
>> -    static int once;
>>         ret = kvm_loongarch_put_regs_core(cs);
>>       if (ret) {
>> @@ -843,14 +842,6 @@ int kvm_arch_put_registers(CPUState *cs, KvmPutState level, Error **errp)
>>           return ret;
>>       }
>>   -    if (!once) {
>> -        ret = kvm_set_pv_features(cs);
>> -        if (ret) {
>> -            return ret;
>> -        }
>> -        once = 1;
>> -    }
>> -
>>       if (level >= KVM_PUT_FULL_STATE) {
>>           /*
>>            * only KVM_PUT_FULL_STATE is required, kvm kernel will clear
>> @@ -1209,6 +1200,12 @@ int kvm_arch_init_vcpu(CPUState *cs)
>>           return ret;
>>       }
>>   +    /* pv_features is a per-vCPU attribute; set it here, once per vCPU. */
>> +    ret = kvm_set_pv_features(cs);
>> +    if (ret < 0) {
>> +        return ret;
>> +    }
> I prefer the old method. The pv_features is one vCPU state instead, and it will be added in vCPU VMState if migration is supported on different host kernel version.
> 
> Here is host feature detection, not vCPU state setting.

Thanks, that makes sense. init_vcpu() is the host-detection path; the
per-vCPU write belongs in kvm_arch_put_registers(). I'll keep it there,
drop the VM-global static guard, and push per-vCPU under
level >= KVM_PUT_FULL_STATE, the same gate kvm_set_stealtime() already
uses just below.

For migration across kernel versions, I'll add env->pv_features as a
sub-section of vmstate_loongarch_cpu (leaving version 4 untouched) so
the value travels with the vCPU.

Thanks,
Tao

> 
> Regards
> Bibo Mao
>> +
>>       ret = kvm_cpu_check_ptw(cs, &local_err);
>>       if (ret < 0) {
>>           error_report_err(local_err);
>>
>

Re: [PATCH] target/loongarch/kvm: set pv features per vCPU in kvm_arch_init_vcpu

Posted by Bibo Mao 2 weeks, 3 days ago

On 2026/7/3 下午2:05, Tao Cui wrote:
> 
> 
> 在 2026/6/25 11:14, Bibo Mao 写道:
>>
>>
>> On 2026/6/25 上午9:53, Tao Cui wrote:
>>> From: Tao Cui <cuitao@kylinos.cn>
>>>
>>> kvm_set_pv_features() programs the KVM_FEATURE cpucfg attribute, which is a
>>> per-vCPU setting. It was called from kvm_arch_put_registers() under a
>>> function-local static guard, so it ran only once for the whole VM and only
>>> the first vCPU actually got its pv features pushed to KVM.
>>>
>>> Move the call to kvm_arch_init_vcpu(), which runs once per vCPU, right
>>> after the pv features are computed, and drop the static guard.
>>>
>>> Signed-off-by: Tao Cui <cuitao@kylinos.cn>
>>> ---
>>>    target/loongarch/kvm/kvm.c | 15 ++++++---------
>>>    1 file changed, 6 insertions(+), 9 deletions(-)
>>>
>>> diff --git a/target/loongarch/kvm/kvm.c b/target/loongarch/kvm/kvm.c
>>> index d6539c12ac..985812c828 100644
>>> --- a/target/loongarch/kvm/kvm.c
>>> +++ b/target/loongarch/kvm/kvm.c
>>> @@ -816,7 +816,6 @@ int kvm_arch_get_registers(CPUState *cs, Error **errp)
>>>    int kvm_arch_put_registers(CPUState *cs, KvmPutState level, Error **errp)
>>>    {
>>>        int ret;
>>> -    static int once;
>>>          ret = kvm_loongarch_put_regs_core(cs);
>>>        if (ret) {
>>> @@ -843,14 +842,6 @@ int kvm_arch_put_registers(CPUState *cs, KvmPutState level, Error **errp)
>>>            return ret;
>>>        }
>>>    -    if (!once) {
>>> -        ret = kvm_set_pv_features(cs);
>>> -        if (ret) {
>>> -            return ret;
>>> -        }
>>> -        once = 1;
>>> -    }
>>> -
>>>        if (level >= KVM_PUT_FULL_STATE) {
>>>            /*
>>>             * only KVM_PUT_FULL_STATE is required, kvm kernel will clear
>>> @@ -1209,6 +1200,12 @@ int kvm_arch_init_vcpu(CPUState *cs)
>>>            return ret;
>>>        }
>>>    +    /* pv_features is a per-vCPU attribute; set it here, once per vCPU. */
>>> +    ret = kvm_set_pv_features(cs);
>>> +    if (ret < 0) {
>>> +        return ret;
>>> +    }
>> I prefer the old method. The pv_features is one vCPU state instead, and it will be added in vCPU VMState if migration is supported on different host kernel version.
>>
>> Here is host feature detection, not vCPU state setting.
> 
> Thanks, that makes sense. init_vcpu() is the host-detection path; the
> per-vCPU write belongs in kvm_arch_put_registers(). I'll keep it there,
> drop the VM-global static guard, and push per-vCPU under
> level >= KVM_PUT_FULL_STATE, the same gate kvm_set_stealtime() already
> uses just below.
> 
> For migration across kernel versions, I'll add env->pv_features as a
> sub-section of vmstate_loongarch_cpu (leaving version 4 untouched) so
> the value travels with the vCPU.
yes, instead pv_features is the value of pseudo register 
CPUCFG_KVM_FEATURE 0x40000004, defined in kernel file 
arch/loongarch/include/uapi/asm/kvm_para.h.

Also also remember to add field no_pv_feature in LoongArchCPU object and 
VirtMachineClass. So that with QEMU 11.1 machine type, no_pv_feature is 
true, with higer version than QEMU 11.1,  no_pv_feature is default false.

Regards
Bibo Mao
> 
> Thanks,
> Tao
> 
>>
>> Regards
>> Bibo Mao
>>> +
>>>        ret = kvm_cpu_check_ptw(cs, &local_err);
>>>        if (ret < 0) {
>>>            error_report_err(local_err);
>>>
>>
>