Re: [PATCH v6 13/19] i386: prefer system KVM_GET_SUPPORTED_HV_CPUID ioctl over vCPU's one

[Vitaly Kuznetsov]

Eduardo Habkost <ehabkost@redhat.com> writes:

> On Thu, Apr 22, 2021 at 06:11:24PM +0200, Vitaly Kuznetsov wrote:
>> KVM_GET_SUPPORTED_HV_CPUID was made a system wide ioctl which can be called
>> prior to creating vCPUs and we are going to use that to expand Hyper-V cpu
>> features early. Use it when it is supported by KVM.
>> 
>> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
>> ---
>>  target/i386/kvm/kvm.c | 17 +++++++++++++----
>>  1 file changed, 13 insertions(+), 4 deletions(-)
>> 
>> diff --git a/target/i386/kvm/kvm.c b/target/i386/kvm/kvm.c
>> index feec9f25ba12..5d08f3a39ef7 100644
>> --- a/target/i386/kvm/kvm.c
>> +++ b/target/i386/kvm/kvm.c
>> @@ -928,7 +928,8 @@ static struct {
>>      },
>>  };
>>  
>> -static struct kvm_cpuid2 *try_get_hv_cpuid(CPUState *cs, int max)
>> +static struct kvm_cpuid2 *try_get_hv_cpuid(CPUState *cs, int max,
>> +                                           bool do_sys_ioctl)
>>  {
>>      struct kvm_cpuid2 *cpuid;
>>      int r, size;
>> @@ -937,7 +938,11 @@ static struct kvm_cpuid2 *try_get_hv_cpuid(CPUState 
>> *cs, int max)
>>      cpuid = g_malloc0(size);
>>      cpuid->nent = max;
>>  
>> -    r = kvm_vcpu_ioctl(cs, KVM_GET_SUPPORTED_HV_CPUID, cpuid);
>> +    if (do_sys_ioctl) {
>> +        r = kvm_ioctl(kvm_state, KVM_GET_SUPPORTED_HV_CPUID, cpuid);
>> +    } else {
>> +        r = kvm_vcpu_ioctl(cs, KVM_GET_SUPPORTED_HV_CPUID, cpuid);
>> +    }
>>      if (r == 0 && cpuid->nent >= max) {
>>          r = -E2BIG;
>>      }
>> @@ -964,13 +969,17 @@ static struct kvm_cpuid2 
>> *get_supported_hv_cpuid(CPUState *cs)
>>      /* 0x40000000..0x40000005, 0x4000000A, 0x40000080..0x40000080 leaves */
>>      int max = 10;
>>      int i;
>> +    bool do_sys_ioctl;
>> +
>> +    do_sys_ioctl =
>> +        kvm_check_extension(kvm_state, KVM_CAP_SYS_HYPERV_CPUID) > 0;
>>  
>>      /*
>>       * When the buffer is too small, KVM_GET_SUPPORTED_HV_CPUID fails with
>>       * -E2BIG, however, it doesn't report back the right size. Keep 
>> increasing
>>       * it and re-trying until we succeed.
>>       */
>> -    while ((cpuid = try_get_hv_cpuid(cs, max)) == NULL) {
>> +    while ((cpuid = try_get_hv_cpuid(cs, max, do_sys_ioctl)) == NULL) {
>>          max++;
>>      }
>>  
>> @@ -980,7 +989,7 @@ static struct kvm_cpuid2 
>> *get_supported_hv_cpuid(CPUState *cs)
>>       * information early, just check for the capability and set the bit
>>       * manually.
>>       */
>> -    if (kvm_check_extension(cs->kvm_state,
>> +    if (!do_sys_ioctl && kvm_check_extension(cs->kvm_state,
>>                              KVM_CAP_HYPERV_ENLIGHTENED_VMCS) > 0) {
>
> Oh, this conditional replaces the comment I suggested in patch
> 10/19.  It makes it obvious that the hack can be deleted if we
> remove support for the VCPU ioctl.
>
> So, when exactly will we be able to delete the VCPU ioctl code
> and support only the system ioctl?

When QEMU drops support for kernels < 5.11? Note, current RHEL8 already
supports system version so we're talking about upstream kernels/Ubuntu
LTS/... 

I remember there was a list of supported kernels for QEMU somewhere but
don't seem to be able to find it quickly, could you maybe point me in
the right direction?

>
> Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
>

Thanks!

>>          for (i = 0; i < cpuid->nent; i++) {
>>              if (cpuid->entries[i].function == HV_CPUID_ENLIGHTMENT_INFO) {
>> -- 
>> 2.30.2
>> 

-- 
Vitaly