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Re: [PATCH v2 1/6] virtio/virtio-pci: Handle extra notification data

From: Jonah Palmer
Subject: Re: [PATCH v2 1/6] virtio/virtio-pci: Handle extra notification data
Date: Thu, 14 Mar 2024 16:23:31 -0400
User-agent: Mozilla Thunderbird

On 3/14/24 3:05 PM, Eugenio Perez Martin wrote:
On Thu, Mar 14, 2024 at 5:06 PM Jonah Palmer <jonah.palmer@oracle.com> wrote:

On 3/14/24 10:55 AM, Eugenio Perez Martin wrote:
On Thu, Mar 14, 2024 at 1:16 PM Jonah Palmer <jonah.palmer@oracle.com> wrote:

On 3/13/24 11:01 PM, Jason Wang wrote:
On Wed, Mar 13, 2024 at 7:55 PM Jonah Palmer <jonah.palmer@oracle.com> wrote:

Add support to virtio-pci devices for handling the extra data sent
from the driver to the device when the VIRTIO_F_NOTIFICATION_DATA
transport feature has been negotiated.

The extra data that's passed to the virtio-pci device when this
feature is enabled varies depending on the device's virtqueue

In a split virtqueue layout, this data includes:
    - upper 16 bits: shadow_avail_idx
    - lower 16 bits: virtqueue index

In a packed virtqueue layout, this data includes:
    - upper 16 bits: 1-bit wrap counter & 15-bit shadow_avail_idx
    - lower 16 bits: virtqueue index

Tested-by: Lei Yang <leiyang@redhat.com>
Reviewed-by: Eugenio Pérez <eperezma@redhat.com>
Signed-off-by: Jonah Palmer <jonah.palmer@oracle.com>
    hw/virtio/virtio-pci.c     | 10 +++++++---
    hw/virtio/virtio.c         | 18 ++++++++++++++++++
    include/hw/virtio/virtio.h |  1 +
    3 files changed, 26 insertions(+), 3 deletions(-)

diff --git a/hw/virtio/virtio-pci.c b/hw/virtio/virtio-pci.c
index cb6940fc0e..0f5c3c3b2f 100644
--- a/hw/virtio/virtio-pci.c
+++ b/hw/virtio/virtio-pci.c
@@ -384,7 +384,7 @@ static void virtio_ioport_write(void *opaque, uint32_t 
addr, uint32_t val)
        VirtIOPCIProxy *proxy = opaque;
        VirtIODevice *vdev = virtio_bus_get_device(&proxy->bus);
-    uint16_t vector;
+    uint16_t vector, vq_idx;
        hwaddr pa;

        switch (addr) {
@@ -408,8 +408,12 @@ static void virtio_ioport_write(void *opaque, uint32_t 
addr, uint32_t val)
                vdev->queue_sel = val;
-        if (val < VIRTIO_QUEUE_MAX) {
-            virtio_queue_notify(vdev, val);
+        vq_idx = val;
+        if (vq_idx < VIRTIO_QUEUE_MAX) {
+            if (virtio_vdev_has_feature(vdev, VIRTIO_F_NOTIFICATION_DATA)) {
+                virtio_queue_set_shadow_avail_data(vdev, val);
+            }
+            virtio_queue_notify(vdev, vq_idx);
        case VIRTIO_PCI_STATUS:
diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
index d229755eae..bcb9e09df0 100644
--- a/hw/virtio/virtio.c
+++ b/hw/virtio/virtio.c
@@ -2255,6 +2255,24 @@ void virtio_queue_set_align(VirtIODevice *vdev, int n, 
int align)

+void virtio_queue_set_shadow_avail_data(VirtIODevice *vdev, uint32_t data)

Maybe I didn't explain well, but I think it is better to pass directly
idx to a VirtQueue *. That way only the caller needs to check for a
valid vq idx, and (my understanding is) the virtio.c interface is
migrating to VirtQueue * use anyway.

Oh, are you saying to just pass in a VirtQueue *vq instead of
VirtIODevice *vdev and get rid of the vq->vring.desc check in the function?

No, that needs to be kept. I meant the access to vdev->vq[i] without
checking for a valid i.

Ahh okay I see what you mean. But I thought the following was checking for a valid VQ index:

if (vq_idx < VIRTIO_QUEUE_MAX)

Of course the virtio device may not have up to VIRTIO_QUEUE_MAX virtqueues, so maybe we should be checking for validity like this?

if (vdev->vq[i].vring.num == 0)

Or was there something else you had in mind? Apologies for the confusion.

You can get the VirtQueue in the caller with virtio_get_queue. Which
also does not check for a valid index, but that way is clearer the
caller needs to check it.

Roger, I'll use this instead for clarity.

As a side note, the check for desc != 0 is widespread in QEMU but the
driver may use 0 address for desc, so it's not 100% valid. But to
change that now requires a deeper change out of the scope of this
series, so let's keep it for now :).

Thanks! >

I'll add it to the todo list =]

+    /* Lower 16 bits is the virtqueue index */
+    uint16_t i = data;
+    VirtQueue *vq = &vdev->vq[i];
+    if (!vq->vring.desc) {
+        return;
+    }
+    if (virtio_vdev_has_feature(vdev, VIRTIO_F_RING_PACKED)) {
+        vq->shadow_avail_wrap_counter = (data >> 31) & 0x1;
+        vq->shadow_avail_idx = (data >> 16) & 0x7FFF;
+    } else {
+        vq->shadow_avail_idx = (data >> 16);

Do we need to do a sanity check for this value?


It can't hurt, right? What kind of check did you have in mind?

if (vq->shadow_avail_idx >= vq->vring.num)

I'm a little bit lost too. shadow_avail_idx can take all uint16_t
values. Maybe you meant checking for a valid vq index, Jason?


Or something else?

+    }
    static void virtio_queue_notify_vq(VirtQueue *vq)
        if (vq->vring.desc && vq->handle_output) {
diff --git a/include/hw/virtio/virtio.h b/include/hw/virtio/virtio.h
index c8f72850bc..53915947a7 100644
--- a/include/hw/virtio/virtio.h
+++ b/include/hw/virtio/virtio.h
@@ -335,6 +335,7 @@ void virtio_queue_update_rings(VirtIODevice *vdev, int n);
    void virtio_init_region_cache(VirtIODevice *vdev, int n);
    void virtio_queue_set_align(VirtIODevice *vdev, int n, int align);
    void virtio_queue_notify(VirtIODevice *vdev, int n);
+void virtio_queue_set_shadow_avail_data(VirtIODevice *vdev, uint32_t data);
    uint16_t virtio_queue_vector(VirtIODevice *vdev, int n);
    void virtio_queue_set_vector(VirtIODevice *vdev, int n, uint16_t vector);
    int virtio_queue_set_host_notifier_mr(VirtIODevice *vdev, int n,

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