linux-kernel - Re: [PATCH v4 5/5] x86, kvm: support vcpu preempted check

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Date:   Mon, 24 Oct 2016 17:14:18 +0200
From:   Radim Krčmář <rkrcmar@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Pan Xinhui <xinhui.pan@...ux.vnet.ibm.com>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        virtualization@...ts.linux-foundation.org,
        linux-s390@...r.kernel.org, xen-devel-request@...ts.xenproject.org,
        kvm@...r.kernel.org, benh@...nel.crashing.org, paulus@...ba.org,
        mpe@...erman.id.au, mingo@...hat.com, peterz@...radead.org,
        paulmck@...ux.vnet.ibm.com, will.deacon@....com,
        kernellwp@...il.com, jgross@...e.com, bsingharora@...il.com,
        boqun.feng@...il.com, borntraeger@...ibm.com
Subject: Re: [PATCH v4 5/5] x86, kvm: support vcpu preempted check

2016-10-24 16:39+0200, Paolo Bonzini:
> On 19/10/2016 19:24, Radim Krčmář wrote:
>>> > +	if (vcpu->arch.st.msr_val & KVM_MSR_ENABLED)
>>> > +		if (kvm_read_guest_cached(vcpu->kvm, &vcpu->arch.st.stime,
>>> > +					&vcpu->arch.st.steal,
>>> > +					sizeof(struct kvm_steal_time)) == 0) {
>>> > +			vcpu->arch.st.steal.preempted = 1;
>>> > +			kvm_write_guest_cached(vcpu->kvm, &vcpu->arch.st.stime,
>>> > +					&vcpu->arch.st.steal,
>>> > +					sizeof(struct kvm_steal_time));
>>> > +		}
>> Please name this block of code.  Something like
>>   kvm_steal_time_set_preempted(vcpu);
> 
> While at it:
> 
> 1) the kvm_read_guest_cached is not necessary.  You can rig the call to
> kvm_write_guest_cached so that it only writes vcpu->arch.st.steal.preempted.

I agree.  kvm_write_guest_cached() always writes from offset 0, so we'd
want a new function that allows to specify a starting offset.

Using cached vcpu->arch.st.steal to avoid the read wouldn't be as good.