Re: [PATCH v2 4/5] KVM: add KVM_USER_EXIT vcpu ioctl for userspace exit

From: "Radim Krčmář" <rkrcmar@redhat.com>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: linux-kernel@vger.kernel.org, kvm@vger.kernel.org
Subject: Re: [PATCH v2 4/5] KVM: add KVM_USER_EXIT vcpu ioctl for userspace exit
Date: Thu, 6 Aug 2015 15:44:15 +0200	[thread overview]
Message-ID: <20150806134415.GB31952@potion.brq.redhat.com> (raw)
In-Reply-To: <55C23B91.4010900@redhat.com>

2015-08-05 18:36+0200, Paolo Bonzini:
> On 05/08/2015 18:33, Radim Krčmář wrote:
>> +4.97 KVM_USER_EXIT
>> +
>> +Capability: KVM_CAP_USER_EXIT
>> +Architectures: x86
>> +Type: vcpu ioctl
>> +Parameters: struct kvm_user_exit (in)
>> +Returns: 0 on success,
>> +         -EFAULT if the parameter couldn't be read,
>> +         -EINVAL if 'reserved' is not zeroed,
>> +
>> +struct kvm_user_exit {
>> +	__u8 reserved[32];
>> +};
> 
> Can we just return EINVAL if the parameter is not NULL?

It complicates handling if we extend the ioctl, but removes the useless
clearing/copying/checking now ...

The two obvious extensions are flags to skip kvm_make_request() or
kvm_vcpu_kick(), both of dubious use.  Another possibility is setting up
conditional exits, but that would be better as a separate control, like
most other sophisticated extensions.

I think that u32 flags would be sufficient -- is casting the 'unsigned
long arg' (data pointer) to a value still an accepted solution?