Re: [Xen-devel] [v3 11/15] Update IRTE according to guest interrupt config changes

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

> From: Wu, Feng
> Sent: Wednesday, June 24, 2015 1:18 PM
> 
> When guest changes its interrupt configuration (such as, vector, etc.)
> for direct-assigned devices, we need to update the associated IRTE
> with the new guest vector, so external interrupts from the assigned
> devices can be injected to guests without VM-Exit.
> 
> For lowest-priority interrupts, we use vector-hashing mechamisn to find
> the destination vCPU. This follows the hardware behavior, since modern
> Intel CPUs use vector hashing to handle the lowest-priority interrupt.
> 
> For multicast/broadcast vCPU, we cannot handle it via interrupt posting,
> still use interrupt remapping.
> 
> Signed-off-by: Feng Wu <feng.wu@xxxxxxxxx>
> ---
> v3:
> - Use bitmap to store the all the possible destination vCPUs of an
> interrupt, then trying to find the right destination from the bitmap
> - Typo and some small changes
> 
>  xen/drivers/passthrough/io.c | 96
> +++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 95 insertions(+), 1 deletion(-)
> 
> diff --git a/xen/drivers/passthrough/io.c b/xen/drivers/passthrough/io.c
> index 9b77334..18e24e1 100644
> --- a/xen/drivers/passthrough/io.c
> +++ b/xen/drivers/passthrough/io.c
> @@ -26,6 +26,7 @@
>  #include <asm/hvm/iommu.h>
>  #include <asm/hvm/support.h>
>  #include <xen/hvm/irq.h>
> +#include <asm/io_apic.h>
> 
>  static DEFINE_PER_CPU(struct list_head, dpci_list);
> 
> @@ -199,6 +200,78 @@ void free_hvm_irq_dpci(struct hvm_irq_dpci *dpci)
>      xfree(dpci);
>  }
> 
> +/*
> + * The purpose of this routine is to find the right destination vCPU for
> + * an interrupt which will be delivered by VT-d posted-interrupt. There
> + * are several cases as below:

If you aim to have this interface common to more usages, don't restrict to
VT-d posted-interrupt which should be just an example.

> + *
> + * - For lowest-priority interrupts, we find the destination vCPU from the
> + *   guest vector using vector-hashing mechanism and return true. This 
> follows
> + *   the hardware behavior, since modern Intel CPUs use vector hashing to
> + *   handle the lowest-priority interrupt.

Does AMD use same hashing mechanism? Can this interface be reused by
other IOMMU type or it's an Intel specific implementation?

> + * - Otherwise, for single destination interrupt, it is straightforward to
> + *   find the destination vCPU and return true.
> + * - For multicast/broadcast vCPU, we cannot handle it via interrupt posting,
> + *   so return false.
> + *
> + *   Here is the details about the vector-hashing mechanism:
> + *   1. For lowest-priority interrupts, store all the possible destination
> + *      vCPUs in an array.
> + *   2. Use "gvec % max number of destination vCPUs" to find the right
> + *      destination vCPU in the array for the lowest-priority interrupt.
> + */
> +static struct vcpu *pi_find_dest_vcpu(struct domain *d, uint8_t dest_id,
> +                                      uint8_t dest_mode, uint8_t 
> delivery_mode,
> +                                      uint8_t gvec)
> +{
> +    unsigned long *dest_vcpu_bitmap = NULL;
> +    unsigned int dest_vcpu_num = 0, idx = 0;
> +    int size = (d->max_vcpus + BITS_PER_LONG - 1) / BITS_PER_LONG;
> +    struct vcpu *v, *dest = NULL;
> +    int i;
> +
> +    dest_vcpu_bitmap = xzalloc_array(unsigned long, size);
> +    if ( !dest_vcpu_bitmap )
> +    {
> +        dprintk(XENLOG_G_INFO,
> +                "dom%d: failed to allocate memory\n", d->domain_id);
> +        return NULL;
> +    }
> +
> +    for_each_vcpu ( d, v )
> +    {
> +        if ( !vlapic_match_dest(vcpu_vlapic(v), NULL, 0,
> +                                dest_id, dest_mode) )
> +            continue;
> +
> +        __set_bit(v->vcpu_id, dest_vcpu_bitmap);
> +        dest_vcpu_num++;
> +    }
> +
> +    if ( delivery_mode == dest_LowestPrio )
> +    {
> +        if (  dest_vcpu_num != 0 )
> +        {

Having 'idx=0' here is more readable than initializing it earlier.

> +            for ( i = 0; i <= gvec % dest_vcpu_num; i++)
> +                idx = find_next_bit(dest_vcpu_bitmap, d->max_vcpus, idx) + 1;
> +            idx--;
> +
> +            BUG_ON(idx >= d->max_vcpus || idx < 0);

idx is unsigned int. can't <0

> +            dest = d->vcpu[idx];
> +        }
> +    }
> +    else if (  dest_vcpu_num == 1 )

a comment would be applausive to explain the condition means
fixed destination, while multicast/broadcast will have num as ZERO.

> +    {
> +        idx = find_first_bit(dest_vcpu_bitmap, d->max_vcpus);
> +        BUG_ON(idx >= d->max_vcpus || idx < 0);
> +        dest = d->vcpu[idx];
> +    }
> +
> +    xfree(dest_vcpu_bitmap);
> +
> +    return dest;
> +}
> +
>  int pt_irq_create_bind(
>      struct domain *d, xen_domctl_bind_pt_irq_t *pt_irq_bind)
>  {
> @@ -257,7 +330,7 @@ int pt_irq_create_bind(
>      {
>      case PT_IRQ_TYPE_MSI:
>      {
> -        uint8_t dest, dest_mode;
> +        uint8_t dest, dest_mode, delivery_mode;
>          int dest_vcpu_id;
> 
>          if ( !(pirq_dpci->flags & HVM_IRQ_DPCI_MAPPED) )
> @@ -330,11 +403,32 @@ int pt_irq_create_bind(
>          /* Calculate dest_vcpu_id for MSI-type pirq migration. */
>          dest = pirq_dpci->gmsi.gflags & VMSI_DEST_ID_MASK;
>          dest_mode = !!(pirq_dpci->gmsi.gflags & VMSI_DM_MASK);
> +        delivery_mode = (pirq_dpci->gmsi.gflags >> GFLAGS_SHIFT_DELIV_MODE) &
> +                        VMSI_DELIV_MASK;
>          dest_vcpu_id = hvm_girq_dest_2_vcpu_id(d, dest, dest_mode);
>          pirq_dpci->gmsi.dest_vcpu_id = dest_vcpu_id;
>          spin_unlock(&d->event_lock);
>          if ( dest_vcpu_id >= 0 )
>              hvm_migrate_pirqs(d->vcpu[dest_vcpu_id]);
> +
> +        /* Use interrupt posting if it is supported */
> +        if ( iommu_intpost )
> +        {
> +            struct vcpu *vcpu = pi_find_dest_vcpu(d, dest, dest_mode,
> +                                        delivery_mode, pirq_dpci->gmsi.gvec);
> +
> +            if ( !vcpu )
> +                dprintk(XENLOG_G_WARNING,
> +                        "dom%u: failed to find the dest vCPU for PI, guest "
> +                        "vector:0x%x use software way to deliver the "
> +                        " interrupts.\n", d->domain_id, 
> pirq_dpci->gmsi.gvec);

If software delivery is a normal behavior, no printk here.

Thanks
Kevin


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