Re: [Xen-devel] [v3 12/15] vmx: posted-interrupt handling when vCPU is blocked

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 24/06/15 06:18, Feng Wu wrote:
> This patch includes the following aspects:
> - Add a global vector to wake up the blocked vCPU
>   when an interrupt is being posted to it (This
>   part was sugguested by Yang Zhang <yang.z.zhang@xxxxxxxxx>).
> - Adds a new per-vCPU tasklet to wakeup the blocked
>   vCPU. It can be used in the case vcpu_unblock
>   cannot be called directly.
> - Define two per-cpu variables:
>       * pi_blocked_vcpu:
>       A list storing the vCPUs which were blocked on this pCPU.
>
>       * pi_blocked_vcpu_lock:
>       The spinlock to protect pi_blocked_vcpu.
>
> Signed-off-by: Feng Wu <feng.wu@xxxxxxxxx>
> ---
> v3:
> - This patch is generated by merging the following three patches in v2:
>    [RFC v2 09/15] Add a new per-vCPU tasklet to wakeup the blocked vCPU
>    [RFC v2 10/15] vmx: Define two per-cpu variables
>    [RFC v2 11/15] vmx: Add a global wake-up vector for VT-d Posted-Interrupts
> - rename 'vcpu_wakeup_tasklet' to 'pi_vcpu_wakeup_tasklet'
> - Move the definition of 'pi_vcpu_wakeup_tasklet' to 'struct arch_vmx_struct'
> - rename 'vcpu_wakeup_tasklet_handler' to 'pi_vcpu_wakeup_tasklet_handler'
> - Make pi_wakeup_interrupt() static
> - Rename 'blocked_vcpu_list' to 'pi_blocked_vcpu_list'
> - move 'pi_blocked_vcpu_list' to 'struct arch_vmx_struct'
> - Rename 'blocked_vcpu' to 'pi_blocked_vcpu'
> - Rename 'blocked_vcpu_lock' to 'pi_blocked_vcpu_lock'
>
>  xen/arch/x86/hvm/vmx/vmcs.c        |  3 +++
>  xen/arch/x86/hvm/vmx/vmx.c         | 54 
> ++++++++++++++++++++++++++++++++++++++
>  xen/include/asm-x86/hvm/hvm.h      |  1 +
>  xen/include/asm-x86/hvm/vmx/vmcs.h |  5 ++++
>  xen/include/asm-x86/hvm/vmx/vmx.h  |  5 ++++
>  5 files changed, 68 insertions(+)
>
> diff --git a/xen/arch/x86/hvm/vmx/vmcs.c b/xen/arch/x86/hvm/vmx/vmcs.c
> index 11dc1b5..0c5ce3f 100644
> --- a/xen/arch/x86/hvm/vmx/vmcs.c
> +++ b/xen/arch/x86/hvm/vmx/vmcs.c
> @@ -631,6 +631,9 @@ int vmx_cpu_up(void)
>      if ( cpu_has_vmx_vpid )
>          vpid_sync_all();
>  
> +    INIT_LIST_HEAD(&per_cpu(pi_blocked_vcpu, cpu));
> +    spin_lock_init(&per_cpu(pi_blocked_vcpu_lock, cpu));
> +
>      return 0;
>  }
>  
> diff --git a/xen/arch/x86/hvm/vmx/vmx.c b/xen/arch/x86/hvm/vmx/vmx.c
> index b94ef6a..7db6009 100644
> --- a/xen/arch/x86/hvm/vmx/vmx.c
> +++ b/xen/arch/x86/hvm/vmx/vmx.c
> @@ -82,7 +82,20 @@ static int vmx_msr_read_intercept(unsigned int msr, 
> uint64_t *msr_content);
>  static int vmx_msr_write_intercept(unsigned int msr, uint64_t msr_content);
>  static void vmx_invlpg_intercept(unsigned long vaddr);
>  
> +/*
> + * We maintian a per-CPU linked-list of vCPU, so in PI wakeup handler we
> + * can find which vCPU should be waken up.
> + */
> +DEFINE_PER_CPU(struct list_head, pi_blocked_vcpu);
> +DEFINE_PER_CPU(spinlock_t, pi_blocked_vcpu_lock);
> +
>  uint8_t __read_mostly posted_intr_vector;
> +uint8_t __read_mostly pi_wakeup_vector;
> +
> +static void pi_vcpu_wakeup_tasklet_handler(unsigned long arg)
> +{
> +    vcpu_unblock((struct vcpu *)arg);
> +}
>  
>  static int vmx_domain_initialise(struct domain *d)
>  {
> @@ -148,11 +161,19 @@ static int vmx_vcpu_initialise(struct vcpu *v)
>      if ( v->vcpu_id == 0 )
>          v->arch.user_regs.eax = 1;
>  
> +    tasklet_init(
> +        &v->arch.hvm_vmx.pi_vcpu_wakeup_tasklet,
> +        pi_vcpu_wakeup_tasklet_handler,
> +        (unsigned long)v);

c/s f6dd295 indicates that the global tasklet lock causes a bottleneck
when injecting interrupts, and replaced a tasklet with a softirq to fix
the scalability issue.

I would expect exactly the bottleneck to exist here.

> +
> +    INIT_LIST_HEAD(&v->arch.hvm_vmx.pi_blocked_vcpu_list);
> +
>      return 0;
>  }
>  
>  static void vmx_vcpu_destroy(struct vcpu *v)
>  {
> +    tasklet_kill(&v->arch.hvm_vmx.pi_vcpu_wakeup_tasklet);
>      /*
>       * There are cases that domain still remains in log-dirty mode when it is
>       * about to be destroyed (ex, user types 'xl destroy <dom>'), in which 
> case
> @@ -1848,6 +1869,33 @@ static struct hvm_function_table __initdata 
> vmx_function_table = {
>      .enable_msr_exit_interception = vmx_enable_msr_exit_interception,
>  };
>  
> +/*
> + * Handle VT-d posted-interrupt when VCPU is blocked.
> + */
> +static void pi_wakeup_interrupt(struct cpu_user_regs *regs)
> +{
> +    struct arch_vmx_struct *vmx;
> +    unsigned int cpu = smp_processor_id();
> +
> +    spin_lock(&per_cpu(pi_blocked_vcpu_lock, cpu));

this_cpu($foo) should be used in preference to per_cpu($foo, $myself).

However, always hoist repeated uses of this/per_cpu into local
variables, as the compiler is unable to elide repeated accesses (because
of a deliberate anti-optimisation behind the scenes).

spinlock_t *lock = &this_cpu(pi_blocked_vcpu_lock);
list_head *blocked_vcpus = &this_cpu(ps_blocked_vcpu);

~Andrew

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