Re: [PATCH v6 2/8] KVM: arm/arm64: Factor out functionality to get vgic mmio requester_vcpu

[Christoffer Dall <christoffer.dall@linaro.org>]

On Tue, Dec 05, 2017 at 04:46:08PM +0300, Yury Norov wrote:
> On Mon, Dec 04, 2017 at 09:05:00PM +0100, Christoffer Dall wrote:
> > From: Christoffer Dall <christoffer.dall@linaro.org>
> > 
> > We are about to distinguish between userspace accesses and mmio traps
> > for a number of the mmio handlers.  When the requester vcpu is NULL, it
> > mens we are handling a userspace acccess.
> 
> Typo: means?
> 

yes

> > Factor out the functionality to get the request vcpu into its own
> > function, mostly so we have a common place to document the semantics of
> > the return value.
> > 
> > Also take the chance to move the functionality outside of holding a
> > spinlock and instead explicitly disable and enable preemption.  This
> > supports PREEMPT_RT kernels as well.
> > 
> > Acked-by: Marc Zyngier <marc.zyngier@arm.com>
> > Reviewed-by: Andre Przywara <andre.przywara@arm.com>
> > Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>
> > ---
> >  virt/kvm/arm/vgic/vgic-mmio.c | 44 +++++++++++++++++++++++++++----------------
> >  1 file changed, 28 insertions(+), 16 deletions(-)
> > 
> > diff --git a/virt/kvm/arm/vgic/vgic-mmio.c b/virt/kvm/arm/vgic/vgic-mmio.c
> > index deb51ee16a3d..747b0a3b4784 100644
> > --- a/virt/kvm/arm/vgic/vgic-mmio.c
> > +++ b/virt/kvm/arm/vgic/vgic-mmio.c
> > @@ -122,6 +122,27 @@ unsigned long vgic_mmio_read_pending(struct kvm_vcpu *vcpu,
> >  	return value;
> >  }
> >  
> > +/*
> > + * This function will return the VCPU that performed the MMIO access and
> > + * trapped from twithin the VM, and will return NULL if this is a userspace
> 
> Typo: from within?
> 

yes

> > + * access.
> > + *
> > + * We can disable preemption locally around accessing the per-CPU variable,
> > + * and use the resolved vcpu pointer after enabling preemption again, because
> > + * even if the current thread is migrated to another CPU, reading the per-CPU
> > + * value later will give us the same value as we update the per-CPU variable
> > + * in the preempt notifier handlers.
> > + */
> > +static struct kvm_vcpu *vgic_get_mmio_requester_vcpu(void)
> > +{
> > +	struct kvm_vcpu *vcpu;
> > +
> > +	preempt_disable();
> > +	vcpu = kvm_arm_get_running_vcpu();
> > +	preempt_enable();
> > +	return vcpu;
> > +}
> > +
> >  void vgic_mmio_write_spending(struct kvm_vcpu *vcpu,
> >  			      gpa_t addr, unsigned int len,
> >  			      unsigned long val)
> > @@ -184,24 +205,10 @@ unsigned long vgic_mmio_read_active(struct kvm_vcpu *vcpu,
> >  static void vgic_mmio_change_active(struct kvm_vcpu *vcpu, struct vgic_irq *irq,
> >  				    bool new_active_state)
> >  {
> > -	struct kvm_vcpu *requester_vcpu;
> >  	unsigned long flags;
> > -	spin_lock_irqsave(&irq->irq_lock, flags);
> > +	struct kvm_vcpu *requester_vcpu = vgic_get_mmio_requester_vcpu();
> >  
> > -	/*
> > -	 * The vcpu parameter here can mean multiple things depending on how
> > -	 * this function is called; when handling a trap from the kernel it
> > -	 * depends on the GIC version, and these functions are also called as
> > -	 * part of save/restore from userspace.
> > -	 *
> > -	 * Therefore, we have to figure out the requester in a reliable way.
> > -	 *
> > -	 * When accessing VGIC state from user space, the requester_vcpu is
> > -	 * NULL, which is fine, because we guarantee that no VCPUs are running
> > -	 * when accessing VGIC state from user space so irq->vcpu->cpu is
> > -	 * always -1.
> > -	 */
> > -	requester_vcpu = kvm_arm_get_running_vcpu();
> > +	spin_lock_irqsave(&irq->irq_lock, flags);
> >  
> >  	/*
> >  	 * If this virtual IRQ was written into a list register, we
> > @@ -213,6 +220,11 @@ static void vgic_mmio_change_active(struct kvm_vcpu *vcpu, struct vgic_irq *irq,
> >  	 * vgic_change_active_prepare)  and still has to sync back this IRQ,
> >  	 * so we release and re-acquire the spin_lock to let the other thread
> >  	 * sync back the IRQ.
> > +	 *
> > +	 * When accessing VGIC state from user space, requester_vcpu is
> > +	 * NULL, which is fine, because we guarantee that no VCPUs are running
> > +	 * when accessing VGIC state from user space so irq->vcpu->cpu is
> > +	 * always -1.
> >  	 */
> >  	while (irq->vcpu && /* IRQ may have state in an LR somewhere */
> >  	       irq->vcpu != requester_vcpu && /* Current thread is not the VCPU thread */
> > -- 
> > 2.14.2

Thanks,
-Christoffer