> On Feb 11, 2022, at 5:31 AM, Thanos Makatos <thanos.maka...@nutanix.com>
> wrote:
>
>
>> -----Original Message-----
>> From: Qemu-devel <qemu-devel-
>> bounces+thanos.makatos=nutanix....@nongnu.org> On Behalf Of John
>> Johnson
>> Sent: 12 January 2022 00:44
>> To: qemu-devel@nongnu.org
>> Subject: [RFC v4 20/21] vfio-user: migration support
>>
>>
>> +static int vfio_user_dirty_bitmap(VFIOProxy *proxy,
>> + struct vfio_iommu_type1_dirty_bitmap *cmd,
>> + struct vfio_iommu_type1_dirty_bitmap_get
>> + *dbitmap)
>> +{
>> + g_autofree struct {
>> + VFIOUserDirtyPages msg;
>> + VFIOUserBitmapRange range;
>> + } *msgp = NULL;
>> + int msize, rsize;
>> +
>> + /*
>> + * If just the command is sent, the returned bitmap isn't needed.
>> + * The bitmap structs are different from the ioctl() versions,
>> + * ioctl() returns the bitmap in a local VA
>> + */
>> + if (dbitmap != NULL) {
>> + msize = sizeof(*msgp);
>> + rsize = msize + dbitmap->bitmap.size;
>> + msgp = g_malloc0(rsize);
>> + msgp->range.iova = dbitmap->iova;
>> + msgp->range.size = dbitmap->size;
>> + msgp->range.bitmap.pgsize = dbitmap->bitmap.pgsize;
>> + msgp->range.bitmap.size = dbitmap->bitmap.size;
>> + } else {
>> + msize = rsize = sizeof(VFIOUserDirtyPages);
>> + msgp = g_malloc0(rsize);
>> + }
>> +
>> + vfio_user_request_msg(&msgp->msg.hdr, VFIO_USER_DIRTY_PAGES, msize,
>> 0);
>> + msgp->msg.argsz = rsize - sizeof(VFIOUserHdr);
>> + msgp->msg.flags = cmd->flags;
>> +
>> + vfio_user_send_wait(proxy, &msgp->msg.hdr, NULL, rsize, false);
>> + if (msgp->msg.hdr.flags & VFIO_USER_ERROR) {
>> + return -msgp->msg.hdr.error_reply;
>> + }
>
> We need to check argsz in the response, in which case the client needs to
> retry with a larger argsz.
>
The client needs to retry if the server reply can be larger than the
client
expects, such as GET_REGION_INFO, where the client doesn’t know how many
capabilities
will be returned a priori.
In this case, argsz is derived from dbitmap->bitmap.size, which was set
in
vfio_get_dirty_bitmap() to be large enough to cover the entire range:
pages = REAL_HOST_PAGE_ALIGN(range->size) / qemu_real_host_page_size;
range->bitmap.size = ROUND_UP(pages, sizeof(__u64) * BITS_PER_BYTE) /
BITS_PER_BYTE;
so I’m not sure we’d ever see a case where the reply is larger than expected.
JJ
>> +
>> + if (dbitmap != NULL) {
>> + memcpy(dbitmap->bitmap.data, &msgp->range.bitmap.data,
>> + dbitmap->bitmap.size);
>> + }
>> +
>> + return 0;
>> +}
>> +
>>
