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

Yury Norov
2017-12-05T13:43:27Z

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?

> 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?

> + * 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