On Thu, 5 Dec 2024 at 15:00, Richard Patel <ripatel@xxxxxxx> wrote: > > Hello, > > Is it safe to share the same virtual memory region as UMEM between two zero-copy XSKs without setting XDP_SHARED_UMEM? > The XSKs do not share the same (dev, queue) pair. They are backed by separate receive queues and have separate fill and completion rings. It is safe to share a umem by just registering the same umem region for each socket. This was the method before XDP_SHARED_UMEM. The drawback is that it consumes more memory as the xsk code in the kernel does not understand that it is indeed shared and structures could be shared too. > I have an application receiving and sending packets via two such XSKs on the same thread. > I’d like to reuse buffers across both XSKs while ensuring each chunk is only submitted to one of the fill or TX rings. > > In principle, this seems safe but undocumented. > If queue IDs or devices between two XSKs differ, the only notable difference in behavior when setting XDP_SHARED_UMEM is that xp_assign_dev_shared is called instead of xp_assign_dev. However, xdp_assign_dev_shared only copies XSK flags and then tail calls to xp_assign_dev. > > Is there any special behaviour introduced by XDP_SHARED_UMEM in this case that I’m missing? > (Perhaps something to do with ref counting of memory pinning, or support for simultaneous DMAs from different devices into the same page?) > And is it possible that future changes to the kernel AF_XDP code or kernel drivers might break such operation without XDP_SHARED_UMEM? > > Thanks > Richard
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