Re: [Qemu-devel] [PATCH] i386/kvm: add support for KVM_CAP_X86_DISABLE_EXITS

[Eduardo Habkost <ehabkost@redhat.com>]

On Fri, Mar 16, 2018 at 07:36:42AM -0700, Wanpeng Li wrote:
> From: Wanpeng Li <wanpengli@tencent.com>
> 
> This patch adds support for KVM_CAP_X86_DISABLE_EXITS. Provides userspace with 
> per-VM capability(KVM_CAP_X86_DISABLE_EXITS) to not intercept MWAIT/HLT/PAUSE 
> in order that to improve latency in some workloads.
> 
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: Radim Krčmář <rkrcmar@redhat.com>
> Cc: Eduardo Habkost <ehabkost@redhat.com>
> Signed-off-by: Wanpeng Li <wanpengli@tencent.com>


Thanks.

Patch looks good (except for comment below), but I would like to
see QEMU documentation mentioning what exactly are the practical
consequences of setting "+kvm-hint-dedicated" (especially what
could happen if people enable the flag without properly
configuring vCPU pinning).


[...]
> +    if (env->features[FEAT_KVM_HINTS] & KVM_HINTS_DEDICATED) {
> +        int disable_exits = kvm_check_extension(cs->kvm_state, KVM_CAP_X86_DISABLE_EXITS);
> +        if (disable_exits) {
> +            disable_exits &= (KVM_X86_DISABLE_EXITS_MWAIT |
> +                              KVM_X86_DISABLE_EXITS_HLT |
> +                              KVM_X86_DISABLE_EXITS_PAUSE);
> +        }

Documentation/virtual/kvm/api.txt says that KVM_FEATURE_PV_UNHALT
shouldn't be enabled if disabling HLT exits.  This needs to be
handled by QEMU.

Probably the simplest solution is to not allow kvm-hint-dedicated
to be enabled if kvm-pv-unhalt is.  This should be mentioned in
QEMU documentation, also, especially considering that we might
enable kvm-pv-unhalt by default in future QEMU versions.


> +        if (kvm_vm_enable_cap(cs->kvm_state, KVM_CAP_X86_DISABLE_EXITS, 0, disable_exits)) {
> +            error_report("kvm: DISABLE EXITS not supported");
> +        }
> +    }
> +
>      qemu_add_vm_change_state_handler(cpu_update_state, env);
>  
>      c = cpuid_find_entry(&cpuid_data.cpuid, 1, 0);
> -- 
> 2.7.4
> 
> 

-- 
Eduardo