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

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Message-ID: <20150806172107.GB1481@potion.brq.redhat.com>
Date:	Thu, 6 Aug 2015 19:21:08 +0200
From:	Radim Krčmář <rkrcmar@...hat.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH v2 4/5] KVM: add KVM_USER_EXIT vcpu ioctl for userspace
 exit

2015-08-06 15:52+0200, Paolo Bonzini:
> On 06/08/2015 15:44, Radim Krčmář wrote:
>> The two obvious extensions are flags to skip kvm_make_request() or
>> kvm_vcpu_kick(), both of dubious use.
> 
> Skipping kvm_make_request() would make some sense if you can set
> vcpu->run->request_interrupt_window asynchronously.  So you could do
> 
> 	vcpu->run->request_interrupt_window = 1;
> 	ioctl(vcpu_fd, KVM_USER_EXIT, KVM_USER_EXIT_LAZY);
> 
> and only cause a lightweight vmexit if the interrupt window is currently
> closed.  I haven't thought of any races that could happen, but it looks
> like it could work.

Seems doable, kvm_run should have been better protected :)

> Skipping kvm_vcpu_kick() makes much less sense.

Could save some cycles when queuing an early exit from the VCPU thread.

>> 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?
> 
> Yeah, that would work for me as well.  Also because, for now, you'd
> return EINVAL if the unsigned long is not zero, which boils down to
> "return EINVAL if the parameter is not NULL". :)

Exactly, only the ioctl number will change.
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