Re: [virtio-comment] Hardware friendly proposals from Intel for packed-ring-layout

["Steven Luong (sluong)" <sluong@cisco.com>]

If the tail pointer is available to the device, can an implementation not check the DESC_HW flag on each individual descriptor to save some cycle? Implementation is supposed to check VRING_DESC_F_WRITE prior to the write. However, I don’t see either DPDK or VPP is checking it. I don’t know if they are doing that on purpose to save some cycle or an oversight.

Steven

On 8/24/17, 7:25 AM, "virtio-comment@lists.oasis-open.org on behalf of Paolo Bonzini" <virtio-comment@lists.oasis-open.org on behalf of pbonzini@redhat.com> wrote:

    On 24/08/2017 16:23, Michael S. Tsirkin wrote:
    > On Thu, Aug 24, 2017 at 02:10:34PM +0200, Paolo Bonzini wrote:
    >> On 24/08/2017 13:53, Tiwei Bie wrote:
    >>>
    >>> * In addition to the DESC_HW flag, each virtio queue has a tail pointer
    >>>     - Driver creates suitable (i.e. multiple of cacheline) descriptors,
    >>>       then performs MMIO write to tail pointer.
    >>
    >> If I understand correctly, the tail pointer is the value that is written
    >> to the MMIO register.  If that is the case, this is unfortunately bad
    >> for virtualization.  Virt prefers a doorbell register where the value
    >> doesn't matter.
    > 
    > I think it's fine actually, this tail pointer merely duplicates data
    > that is already available in form of DESC_HW flag.  Virt can just ignore
    > the value.
    
    If the spec says the hardware can read speculatively beyond the tail
    pointer, that's fine indeed (that was not in Tiwei's proposal).
    
    Paolo
    
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