Re: [Qemu-devel] [PATCH v2 3/4] vfio/quirks: ioeventfd quirk acceleration

[Peter Xu <peterx@redhat.com>]

On Tue, May 01, 2018 at 10:43:32AM -0600, Alex Williamson wrote:

[...]

> @@ -743,6 +843,60 @@ static void vfio_nvidia_quirk_mirror_write(void *opaque, hwaddr addr,
>                            addr + mirror->offset, data, size);
>          trace_vfio_quirk_nvidia_bar0_msi_ack(vdev->vbasedev.name);
>      }
> +
> +    /*
> +     * Automatically add an ioeventfd to handle any repeated write with the
> +     * same data and size above the standard PCI config space header.  This is
> +     * primarily expected to accelerate the MSI-ACK behavior, such as noted
> +     * above.  Current hardware/drivers should trigger an ioeventfd at config
> +     * offset 0x704 (region offset 0x88704), with data 0x0, size 4.
> +     *
> +     * The criteria of 10 successive hits is arbitrary but reliably adds the
> +     * MSI-ACK region.  Note that as some writes are bypassed via the ioeventfd,
> +     * the remaining ones have a greater chance of being seen successively.
> +     * To avoid the pathological case of burning up all of QEMU's open file
> +     * handles, arbitrarily limit this algorithm from adding no more than 10
> +     * ioeventfds, print an error if we would have added an 11th, and then
> +     * stop counting.
> +     */
> +    if (!vdev->no_kvm_ioeventfd &&
> +        addr > PCI_STD_HEADER_SIZEOF && last->added < MAX_DYN_IOEVENTFD + 1) {
> +        if (addr != last->addr || data != last->data || size != last->size) {
> +            last->addr = addr;
> +            last->data = data;
> +            last->size = size;
> +            last->hits = 1;
> +        } else if (++last->hits >= HITS_FOR_IOEVENTFD) {
> +            if (last->added < MAX_DYN_IOEVENTFD) {
> +                VFIOIOEventFD *ioeventfd;
> +                ioeventfd = vfio_ioeventfd_init(vdev, mirror->mem, addr, size,
> +                                        data, &vdev->bars[mirror->bar].region,
> +                                        mirror->offset + addr, true);
> +                if (ioeventfd) {
> +                    VFIOQuirk *quirk;
> +
> +                    QLIST_FOREACH(quirk,
> +                                  &vdev->bars[mirror->bar].quirks, next) {

I'm not sure whether the quirks list can be a long one, otherwise not
sure whether we can just cache the quirk pointer inside the mirror to
avoid the loop.

> +                        if (quirk->data == mirror) {
> +                            QLIST_INSERT_HEAD(&quirk->ioeventfds,
> +                                              ioeventfd, next);
> +                            break;
> +                        }
> +                    }

[...]

> +typedef struct VFIOIOEventFD {
> +    QLIST_ENTRY(VFIOIOEventFD) next;
> +    MemoryRegion *mr;
> +    hwaddr addr;
> +    unsigned size;
> +    uint64_t data;
> +    EventNotifier e;
> +    VFIORegion *region;
> +    hwaddr region_addr;
> +    bool match_data;

Would it possible in the future that match_data will be false?
Otherwise I'm not sure whether we can even omit this field.

> +    bool dynamic;
> +} VFIOIOEventFD;
> +

The two comments are only questions I thought about.  No matter what
the patch looks quite good to me already, so:

Reviewed-by: Peter Xu <peterx@redhat.com>

Regards,

-- 
Peter Xu