Re: [Xen-devel] [PATCH v5 7/7] VMX: Fixup PI descriptor when cpu is offline

["Tian, Kevin" <kevin.tian@xxxxxxxxx>]

> From: Wu, Feng
> Sent: Tuesday, October 11, 2016 8:58 AM
> 
> When cpu is offline, we need to move all the vcpus in its blocking
> list to another online cpu, this patch handles it.
> 
> Signed-off-by: Feng Wu <feng.wu@xxxxxxxxx>
> ---
> v5:
> - Add some comments to explain why it doesn't cause deadlock
> for the ABBA deadlock scenario.
> 
>  xen/arch/x86/hvm/vmx/vmcs.c       |  1 +
>  xen/arch/x86/hvm/vmx/vmx.c        | 48
> +++++++++++++++++++++++++++++++++++++++
>  xen/include/asm-x86/hvm/vmx/vmx.h |  1 +
>  3 files changed, 50 insertions(+)
> 
> diff --git a/xen/arch/x86/hvm/vmx/vmcs.c b/xen/arch/x86/hvm/vmx/vmcs.c
> index 10976bd..5dd68ca 100644
> --- a/xen/arch/x86/hvm/vmx/vmcs.c
> +++ b/xen/arch/x86/hvm/vmx/vmcs.c
> @@ -578,6 +578,7 @@ void vmx_cpu_dead(unsigned int cpu)
>      vmx_free_vmcs(per_cpu(vmxon_region, cpu));
>      per_cpu(vmxon_region, cpu) = 0;
>      nvmx_cpu_dead(cpu);
> +    vmx_pi_desc_fixup(cpu);
>  }
> 
>  int vmx_cpu_up(void)
> diff --git a/xen/arch/x86/hvm/vmx/vmx.c b/xen/arch/x86/hvm/vmx/vmx.c
> index b14c84e..c71d496 100644
> --- a/xen/arch/x86/hvm/vmx/vmx.c
> +++ b/xen/arch/x86/hvm/vmx/vmx.c
> @@ -208,6 +208,54 @@ static void vmx_pi_do_resume(struct vcpu *v)
>      vmx_pi_list_remove(v);
>  }
> 
> +void vmx_pi_desc_fixup(int cpu)
> +{
> +    unsigned int new_cpu, dest;
> +    unsigned long flags;
> +    struct arch_vmx_struct *vmx, *tmp;
> +    spinlock_t *new_lock, *old_lock = &per_cpu(vmx_pi_blocking, cpu).lock;
> +    struct list_head *blocked_vcpus = &per_cpu(vmx_pi_blocking, cpu).list;
> +
> +    if ( !iommu_intpost )
> +        return;
> +
> +    /*
> +     * We are in the context of CPU_DEAD or CPU_UP_CANCELED notification,
> +     * and it is impossible for a second CPU go down in parallel. So we
> +     * can safely acquire the old cpu's lock and then acquire the new_cpu's
> +     * lock after that.
> +     */
> +    spin_lock_irqsave(old_lock, flags);
> +
> +    list_for_each_entry_safe(vmx, tmp, blocked_vcpus, pi_blocking.list)
> +    {
> +        /*
> +         * We need to find an online cpu as the NDST of the PI descriptor, it
> +         * doesn't matter whether it is within the cpupool of the domain or
> +         * not. As long as it is online, the vCPU will be woken up once the
> +         * notification event arrives.
> +         */
> +        new_cpu = cpumask_any(&cpu_online_map);
> +        new_lock = &per_cpu(vmx_pi_blocking, new_cpu).lock;
> +
> +        spin_lock(new_lock);
> +
> +        ASSERT(vmx->pi_blocking.lock == old_lock);
> +
> +        dest = cpu_physical_id(new_cpu);
> +        write_atomic(&vmx->pi_desc.ndst,
> +                     x2apic_enabled ? dest : MASK_INSR(dest, 
> PI_xAPIC_NDST_MASK));
> +
> +        list_move(&vmx->pi_blocking.list,
> +                  &per_cpu(vmx_pi_blocking, new_cpu).list);
> +        vmx->pi_blocking.lock = new_lock;
> +
> +        spin_unlock(new_lock);

I didn't check the whole flow... but did you suppress notification somewhere
earlier before above list movement happens? Otherwise you may miss an
interrupt when the target cpu is dying...

> +    }
> +
> +    spin_unlock_irqrestore(old_lock, flags);
> +}
> +
>  /* This function is called when pcidevs_lock is held */
>  void vmx_pi_hooks_assign(struct domain *d)
>  {
> diff --git a/xen/include/asm-x86/hvm/vmx/vmx.h
> b/xen/include/asm-x86/hvm/vmx/vmx.h
> index 4cdd9b1..9783c70 100644
> --- a/xen/include/asm-x86/hvm/vmx/vmx.h
> +++ b/xen/include/asm-x86/hvm/vmx/vmx.h
> @@ -569,6 +569,7 @@ void free_p2m_hap_data(struct p2m_domain *p2m);
>  void p2m_init_hap_data(struct p2m_domain *p2m);
> 
>  void vmx_pi_per_cpu_init(unsigned int cpu);
> +void vmx_pi_desc_fixup(int cpu);
> 
>  void vmx_pi_hooks_assign(struct domain *d);
>  void vmx_pi_hooks_deassign(struct domain *d);
> --
> 2.1.0


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