Subject: [PATCH v3 14/16] KVM: Move vcpu_load to arch-specific kvm_arch_vcpu_ioctl

From: [email-masked] (Christoffer Dall)
Date: Mon, 11 Dec 2017 16:22:26 +0100

On Mon, Dec 11, 2017 at 02:12:41PM +0100, Cornelia Huck wrote:
> On Mon,  4 Dec 2017 21:35:36 +0100
> Christoffer Dall <[email-masked]> wrote:
> 
> > From: Christoffer Dall <[email-masked]>
> > 
> > Move the calls to vcpu_load() and vcpu_put() in to the architecture
> > specific implementations of kvm_arch_vcpu_ioctl() which dispatches
> > further architecture-specific ioctls on to other functions.
> > 
> > Some architectures support asynchronous vcpu ioctls which cannot call
> > vcpu_load() or take the vcpu->mutex, because that would prevent
> > concurrent execution with a running VCPU, which is the intended purpose
> > of these ioctls, for example because they inject interrupts.
> > 
> > We repeat the separate checks for these specifics in the architecture
> > code for MIPS, S390 and PPC, and avoid taking the vcpu->mutex and
> > calling vcpu_load for these ioctls.
> > 
> > Signed-off-by: Christoffer Dall <[email-masked]>
> > ---
> >  arch/mips/kvm/mips.c       | 49 +++++++++++++++++++++++----------------
> >  arch/powerpc/kvm/powerpc.c | 13 ++++++-----
> >  arch/s390/kvm/kvm-s390.c   | 19 ++++++++-------
> >  arch/x86/kvm/x86.c         | 22 +++++++++++++-----
> >  virt/kvm/arm/arm.c         | 58 ++++++++++++++++++++++++++++++++--------------
> >  virt/kvm/kvm_main.c        |  2 --
> >  6 files changed, 103 insertions(+), 60 deletions(-)
> > 
> > diff --git a/arch/mips/kvm/mips.c b/arch/mips/kvm/mips.c
> > index 3a898712d6cd..4a039341dc29 100644
> > --- a/arch/mips/kvm/mips.c
> > +++ b/arch/mips/kvm/mips.c
> > @@ -913,56 +913,65 @@ long kvm_arch_vcpu_ioctl(struct file *filp, unsigned int ioctl,
> >  	void __user *argp = (void __user *)arg;
> >  	long r;
> >  
> > +	if (ioctl == KVM_INTERRUPT) {
> 
> I would add a comment here that this ioctl is async to vcpu execution,
> so it is understandable why you skip the vcpu_load().

Yes, that would be appropriate.

> 
> [As an aside, it is nice that this is now more obvious when looking at
> the architectures' handlers.]
> 

Agreed.

> > +		struct kvm_mips_interrupt irq;
> > +
> > +		if (copy_from_user(&irq, argp, sizeof(irq)))
> > +			return -EFAULT;
> > +		kvm_debug("[%d] %s: irq: %d\n", vcpu->vcpu_id, __func__,
> > +			  irq.irq);
> > +
> > +		return kvm_vcpu_ioctl_interrupt(vcpu, &irq);
> > +	}
> > +
> > +	vcpu_load(vcpu);
> > +
> >  	switch (ioctl) {
> 
> (...)
> 
> > diff --git a/arch/powerpc/kvm/powerpc.c b/arch/powerpc/kvm/powerpc.c
> > index c06bc9552438..6b5dd3a25fe8 100644
> > --- a/arch/powerpc/kvm/powerpc.c
> > +++ b/arch/powerpc/kvm/powerpc.c
> > @@ -1617,16 +1617,16 @@ long kvm_arch_vcpu_ioctl(struct file *filp,
> >  	void __user *argp = (void __user *)arg;
> >  	long r;
> >  
> > -	switch (ioctl) {
> > -	case KVM_INTERRUPT: {
> > +	if (ioctl == KVM_INTERRUPT) {
> 
> Same here.
> 
> >  		struct kvm_interrupt irq;
> > -		r = -EFAULT;
> >  		if (copy_from_user(&irq, argp, sizeof(irq)))
> > -			goto out;
> > -		r = kvm_vcpu_ioctl_interrupt(vcpu, &irq);
> > -		goto out;
> > +			return -EFAULT;
> > +		return kvm_vcpu_ioctl_interrupt(vcpu, &irq);
> >  	}
> >  
> > +	vcpu_load(vcpu);
> > +
> > +	switch (ioctl) {
> >  	case KVM_ENABLE_CAP:
> >  	{
> >  		struct kvm_enable_cap cap;
> > @@ -1666,6 +1666,7 @@ long kvm_arch_vcpu_ioctl(struct file *filp,
> >  	}
> >  
> >  out:
> > +	vcpu_put(vcpu);
> >  	return r;
> >  }
> >  
> > diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
> > index 43278f334ce3..cd067b63d77f 100644
> > --- a/arch/s390/kvm/kvm-s390.c
> > +++ b/arch/s390/kvm/kvm-s390.c
> > @@ -3743,24 +3743,25 @@ long kvm_arch_vcpu_ioctl(struct file *filp,
> >  	case KVM_S390_IRQ: {
> >  		struct kvm_s390_irq s390irq;
> >  
> > -		r = -EFAULT;
> >  		if (copy_from_user(&s390irq, argp, sizeof(s390irq)))
> > -			break;
> > -		r = kvm_s390_inject_vcpu(vcpu, &s390irq);
> > -		break;
> > +			return -EFAULT;
> > +		return kvm_s390_inject_vcpu(vcpu, &s390irq);
> >  	}
> >  	case KVM_S390_INTERRUPT: {
> >  		struct kvm_s390_interrupt s390int;
> >  		struct kvm_s390_irq s390irq;
> >  
> > -		r = -EFAULT;
> >  		if (copy_from_user(&s390int, argp, sizeof(s390int)))
> > -			break;
> > +			return -EFAULT;
> >  		if (s390int_to_s390irq(&s390int, &s390irq))
> >  			return -EINVAL;
> > -		r = kvm_s390_inject_vcpu(vcpu, &s390irq);
> > -		break;
> > +		return kvm_s390_inject_vcpu(vcpu, &s390irq);
> >  	}
> > +	}
> 
> I find the special casing with the immediate return a bit ugly. Maybe
> introduce a helper async_vcpu_ioctl() or so that sets -ENOIOCTLCMD in
> its default case and return here if ret != -ENOIOCTLCMD? Christian,
> what do you think?
> 
> > +
> > +	vcpu_load(vcpu);
> > +
> > +	switch (ioctl) {
> >  	case KVM_S390_STORE_STATUS:
> >  		idx = srcu_read_lock(&vcpu->kvm->srcu);
> >  		r = kvm_s390_vcpu_store_status(vcpu, arg);
> > @@ -3883,6 +3884,8 @@ long kvm_arch_vcpu_ioctl(struct file *filp,
> >  	default:
> >  		r = -ENOTTY;
> >  	}
> > +
> > +	vcpu_put(vcpu);
> >  	return r;
> >  }
> >  
> 
> (...)
> 
> > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > index 06751bbecd58..ad5f83159a15 100644
> > --- a/virt/kvm/kvm_main.c
> > +++ b/virt/kvm/kvm_main.c
> > @@ -2693,9 +2693,7 @@ static long kvm_vcpu_ioctl(struct file *filp,
> >  		break;
> >  	}
> >  	default:
> > -		vcpu_load(vcpu);
> >  		r = kvm_arch_vcpu_ioctl(filp, ioctl, arg);
> > -		vcpu_put(vcpu);
> >  	}
> >  out:
> >  	mutex_unlock(&vcpu->mutex);
> 
> It would be nice if we could get rid of the special casing at the
> beginning of this function, but as it would involve not taking the
> mutex for special cases (and not releasing it for those special cases),
> I don't see an elegant way to do that.

I would also have liked that, and that's essentially what I had in the
first version, but Paolo thought the result was too high an increase in
complexity in the architecture-specfic functions throughout.  I don't
have any better suggestions either.

Thanks,
-Christoffer