linux-kernel - Re: [PATCH v5 4/9] KVM-HV: KVM Steal time implementation

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Date:	Mon, 11 Jul 2011 15:58:18 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Glauber Costa <glommer@...hat.com>
CC:	Marcelo Tosatti <mtosatti@...hat.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, Rik van Riel <riel@...hat.com>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Anthony Liguori <aliguori@...ibm.com>,
	Eric B Munson <emunson@...bm.net>
Subject: Re: [PATCH v5 4/9] KVM-HV: KVM Steal time implementation

On 07/07/2011 08:07 PM, Glauber Costa wrote:
>>> +static void record_steal_time(struct kvm_vcpu *vcpu)
>>> +{
>>> +    u64 delta;
>>> +
>>> +    if (!(vcpu->arch.st.msr_val&  KVM_MSR_ENABLED))
>>> +        return;
>>> +
>>> +    if (unlikely(kvm_read_guest_cached(vcpu->kvm,&vcpu->arch.st.stime,
>>> + &vcpu->arch.st.steal, sizeof(struct kvm_steal_time))))
>>> +        return;
>>
>> The guest memory page is not pinned, sleeping via
>> __copy_from_user/to_user is not allowed in vcpu_load context. Either pin
>> it or use atomic acessors.
>
>
> I do recognize the problem.
> Avi, what's your take here?
>

The easiest solution is to set a KVM_REQ bit in atomic context, and move 
the sleepy code to vcpu_enter_guest().

>>> +    case MSR_KVM_STEAL_TIME:
>>> +        vcpu->arch.st.msr_val = data;
>>> +
>>> +        if (!(data&  KVM_MSR_ENABLED)) {
>>> +            break;
>>> +        }
>>
>> On failure below this point, msr_val should be cleared of 
>> KVM_MSR_ENABLED?
> No, msr_val has to hold whatever the guest wrote into it.
> We should probably use an independent variable here to indicate that 
> we failed to activate it.

If we fail, we return a #GP to the guest (and don't write any value into 
the msr).

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.

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