Virtualization - Jan 2022 - [PATCH 2/6] virtio: split: alloc indirect desc with extra

? 2022/1/7 ??2:33, Xuan Zhuo ??:
> In the scenario where indirect is not used, each desc corresponds to an
> extra, which is used to record information such as dma, flags, and
> next.
>
> In the scenario of using indirect, the assigned desc does not have the
> corresponding extra record dma information, and the dma information must
> be obtained from the desc when unmap.
>
> This patch allocates the corresponding extra array when indirect desc is
> allocated. This has these advantages:
> 1. Record the dma information of desc, no need to read desc when unmap
> 2. It will be more convenient and unified in processing
> 3. Some additional information can be recorded in extra, which will be
>     used in subsequent patches.

Two questions:

1) Is there any performance number for this change? I guess it gives 
more stress on the cache.
2) Is there a requirement to mix the pre mapped sg with unmapped sg? If 
not, a per virtqueue flag looks sufficient

Thanks

On Mon, 10 Jan 2022 14:43:39 +0800, Jason Wang <jasowang at redhat.com>
wrote:
>
> ? 2022/1/7 ??2:33, Xuan Zhuo ??:
> > In the scenario where indirect is not used, each desc corresponds to
an
> > extra, which is used to record information such as dma, flags, and
> > next.
> >
> > In the scenario of using indirect, the assigned desc does not have the
> > corresponding extra record dma information, and the dma information
must
> > be obtained from the desc when unmap.
> >
> > This patch allocates the corresponding extra array when indirect desc
is
> > allocated. This has these advantages:
> > 1. Record the dma information of desc, no need to read desc when unmap
> > 2. It will be more convenient and unified in processing
> > 3. Some additional information can be recorded in extra, which will be
> >     used in subsequent patches.
>
>
> Two questions:
>
> 1) Is there any performance number for this change? I guess it gives
> more stress on the cache.
I will add performance test data in the next version.
> 2) Is there a requirement to mix the pre mapped sg with unmapped sg? If
> not, a per virtqueue flag looks sufficient
There is this requirement. For example, in the case of AF_XDP, a patcket
contains two parts, one is virtio_net_hdr, and the other is the actual data
packet from AF_XDP. The former is unmapped sg, and the latter is pre mapped sg.

Thanks.
>
> Thanks
>
>