Re: [RFC v4 20/21] vfio-user: migration support

[John Johnson]

> On Feb 11, 2022, at 5:31 AM, Thanos Makatos <thanos.makatos@nutanix.com> 
> wrote:
> 
> 
>> -----Original Message-----
>> From: Qemu-devel <qemu-devel-
>> bounces+thanos.makatos=nutanix.com@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;
>> +}
>> +
>>