netdev - Re: [PATCH vhost v6 02/10] virtio_ring: packed: remove double check of the unmap ops

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Date: Fri, 29 Mar 2024 11:24:10 +0800
From: Jason Wang <jasowang@...hat.com>
To: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
Cc: virtualization@...ts.linux.dev, "Michael S. Tsirkin" <mst@...hat.com>, 
	"David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, 
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org
Subject: Re: [PATCH vhost v6 02/10] virtio_ring: packed: remove double check
 of the unmap ops

On Thu, Mar 28, 2024 at 4:16 PM Xuan Zhuo <xuanzhuo@...ux.alibaba.com> wrote:
>
> On Thu, 28 Mar 2024 16:07:14 +0800, Jason Wang <jasowang@...hat.com> wrote:
> > On Thu, Mar 28, 2024 at 3:32 PM Xuan Zhuo <xuanzhuo@...ux.alibaba.com> wrote:
> > >
> > > On Thu, 28 Mar 2024 14:56:47 +0800, Jason Wang <jasowang@...hat.com> wrote:
> > > > On Wed, Mar 27, 2024 at 7:14 PM Xuan Zhuo <xuanzhuo@...ux.alibaba.com> wrote:
> > > > >
> > > > > In the functions vring_unmap_extra_packed and vring_unmap_desc_packed,
> > > > > multiple checks are made whether unmap is performed and whether it is
> > > > > INDIRECT.
> > > > >
> > > > > These two functions are usually called in a loop, and we should put the
> > > > > check outside the loop.
> > > > >
> > > > > And we unmap the descs with VRING_DESC_F_INDIRECT on the same path with
> > > > > other descs, that make the thing more complex. If we distinguish the
> > > > > descs with VRING_DESC_F_INDIRECT before unmap, thing will be clearer.
> > > > >
> > > > > For desc with VRING_DESC_F_INDIRECT flag:
> > > > > 1. only one desc of the desc table is used, we do not need the loop
> > > > >     Theoretically, indirect descriptors could be chained.
> > > > >     But now, that is not supported by "add", so we ignore this case.
> > > > > 2. the called unmap api is difference from the other desc
> > > > > 3. the vq->premapped is not needed to check
> > > > > 4. the vq->indirect is not needed to check
> > > > > 5. the state->indir_desc must not be null
> > > >
> > > > It doesn't explain the connection to the goal of this series. If it's
> > > > not a must I'd suggest moving it to a separate patch.
> > >
> > >
> > > The "no store dma ..." depends this.
> > >
> > > I will add this message in next version.
> > >
> > >
> > > >
> > > > >
> > > > > Signed-off-by: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
> > > >
> > > > Rethink this, it looks to me it would complicate the codes furtherly.
> > > >
> > > > For example, vring_map_xxx() helpers will check premappred and
> > > > use_dma_api by itself. But in the case of vring_unmap() you want to
> > > > move those checks to the caller. This will result in tricky codes that
> > > > are hard to understand.
> > > >
> > > > We need to be consistent here.
> > > >
> > > > If we try to optimize unmap we need to optimize map as well. But
> > > > generally it would complicate the logic of the caller if we want to
> > > > let the caller to differ. Ideally, the caller of those function should
> > > > know nothing about use_dma_api, premapped and other.
> > >
> > >
> > > The key is that we can check "use_dma_api, premapped" to skip the loop.
> > > If the vring_unmap_xxx is called, the "use_dma_api, premapped" is checked in
> > > advance, so that is a waste to check thest again.
> >
> > Right, but we have the same logic for map.
>
> But we can not skip the loop for map.

Ok, right. So I'm fine to leave it as is. We can optimize the checking
on top anyhow.

Thanks