[dpdk-dev] [PATCH v2] net/virtio-user: fix not working on 32-bit system

[Tan, Jianfeng]

Hi Jim,

> -----Original Message-----
> From: dev [mailto:dev-bounces at dpdk.org] On Behalf Of Jim Murphy
> Sent: Wednesday, September 20, 2017 6:24 AM
> To: dev at dpdk.org
> Subject: [dpdk-dev] [PATCH v2] net/virtio-user: fix not working on 32-bit
> system
> 
> Hi,
> 
> The fix contained in this patch breaks under the following scenario:
> 
> 1. A 64 bit host and virtual machine. Therefore all physical addresses are
> 64 bits.

Host, VM and application are 64bit, then I suppose there's no problem?

> 2. A 32 bit user mode DPDK process running on a 64 bit virtual machine (64
> bit kernel).

Ah, this is a case we fail to cover.

> 
> In this case, the physical address is 64bits but the virtual address of the
> user process is 32 bits so uintptr_t is only 32 bits. As a result when:
> 
>  (uintptr_t)(mb) + (vq)->offset)
> 
> is referenced, only 32 bits are copied into the descriptor but 64 bits are
> required because in this scenario that is the size of a physical address.
> 
> So it seems like we need a way to determine the size of the physical
> address and then VIRTIO_MBUF_ADDR should be written to copy that many
> bytes
> into the uint64_t. Does anyone know how to determine the size of the
> physical address?

It's easy to find a way to decide the length (similar to _offset_), but copying variable length of bytes seems not an efficient way.

Thanks,
Jianfeng