netdev - Re: [PATCH] vDPA/ifcvf: assign nr_vring to the MSI vector of config

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Message-ID: <CACGkMEuLW_PypyiPVcNz-B5FWNpkzLWwzC0eaZsCih+PbNt3Cg@mail.gmail.com>
Date:   Fri, 15 Apr 2022 13:51:16 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Zhu, Lingshan" <lingshan.zhu@...el.com>
Cc:     mst <mst@...hat.com>,
        virtualization <virtualization@...ts.linux-foundation.org>,
        netdev <netdev@...r.kernel.org>,
        Dan Carpenter <dan.carpenter@...cle.com>
Subject: Re: [PATCH] vDPA/ifcvf: assign nr_vring to the MSI vector of
 config_intr by default

On Wed, Apr 13, 2022 at 5:16 PM Zhu, Lingshan <lingshan.zhu@...el.com> wrote:
>
>
>
> On 4/13/2022 4:14 PM, Jason Wang wrote:
> >
> > 在 2022/4/8 下午8:10, Zhu Lingshan 写道:
> >> This commit assign struct ifcvf_hw.nr_vring to the MSIX vector of the
> >> config interrupt by default in ifcvf_request_config_irq().
> >> ifcvf_hw.nr_vring is the most likely and the ideal case for
> >> the device config interrupt handling, means every virtqueue has
> >> an individual MSIX vector(0 ~ nr_vring - 1), and the config interrupt
> >> has
> >> its own MSIX vector(number nr_vring).
> >>
> >> This change can also make GCC W = 2 happy, silence the
> >> "uninitialized" warning.
> >>
> >> Signed-off-by: Zhu Lingshan <lingshan.zhu@...el.com>
> >> Reviewed-by: Dan Carpenter <dan.carpenter@...cle.com>
> >> ---
> >>   drivers/vdpa/ifcvf/ifcvf_main.c | 8 ++++----
> >>   1 file changed, 4 insertions(+), 4 deletions(-)
> >>
> >> diff --git a/drivers/vdpa/ifcvf/ifcvf_main.c
> >> b/drivers/vdpa/ifcvf/ifcvf_main.c
> >> index 4366320fb68d..b500fb941dab 100644
> >> --- a/drivers/vdpa/ifcvf/ifcvf_main.c
> >> +++ b/drivers/vdpa/ifcvf/ifcvf_main.c
> >> @@ -290,13 +290,13 @@ static int ifcvf_request_config_irq(struct
> >> ifcvf_adapter *adapter)
> >>       struct ifcvf_hw *vf = &adapter->vf;
> >>       int config_vector, ret;
> >>   +    /* vector 0 ~ vf->nr_vring for vqs, num vf->nr_vring vector
> >> for config interrupt */
> >
> >
> > The comment is right before this patch, but probably wrong for
> > MSIX_VECTOR_DEV_SHARED.
> This comment is for the case when every vq and config interrupt has its
> own vector, how
> about a better comment "The ideal the default case, vector 0 ~
> vf->nr_vring for vqs, num vf->nr_vring vector for config interrupt"

Actually, I suggest to remove the comment since the code can explain itself.

> >
> >
> >> +    config_vector = vf->nr_vring;
> >> +
> >> +    /* re-use the vqs vector */
> >>       if (vf->msix_vector_status == MSIX_VECTOR_DEV_SHARED)
> >>           return 0;
> >>   -    if (vf->msix_vector_status == MSIX_VECTOR_PER_VQ_AND_CONFIG)
> >> -        /* vector 0 ~ vf->nr_vring for vqs, num vf->nr_vring vector
> >> for config interrupt */
> >> -        config_vector = vf->nr_vring;
> >> -
> >>       if (vf->msix_vector_status == MSIX_VECTOR_SHARED_VQ_AND_CONFIG)
> >>           /* vector 0 for vqs and 1 for config interrupt */
> >>           config_vector = 1;
> >
> >
> > Actually, I prefer to use if ... else ... here.
> IMHO, if else may lead to mistakes.
>
> The code:
>          /* The ideal the default case, vector 0 ~ vf->nr_vring for vqs,
> num vf->nr_vring vector for config interrupt */
>          config_vector = vf->nr_vring;
>
>          /* re-use the vqs vector */
>          if (vf->msix_vector_status == MSIX_VECTOR_DEV_SHARED)
>                  return 0;
>
>          if (vf->msix_vector_status == MSIX_VECTOR_SHARED_VQ_AND_CONFIG)
>                  /* vector 0 for vqs and 1 for config interrupt */
>                  config_vector = 1;
>
>
> here by default config_vector = vf->nr_vring;
> If msix_vector_status == MSIX_VECTOR_DEV_SHARED, it will reuse the dev
> shared vector, means using the vector(value 0) for data-vqs.
> If msix_vector_status == MSIX_VECTOR_SHARED_VQ_AND_CONFIG, it will use
> vector=1(vector 0 for data-vqs).
>
> If we use if...else, it will be:
>
>          /* re-use the vqs vector */
>          if (vf->msix_vector_status == MSIX_VECTOR_DEV_SHARED)
>                  return 0;
>          else
>                  config_vector = 1;
>
> This looks like config_vector can only be 0(re-used vector for the
> data-vqs, which is 0) or 1. It shadows the ideal and default case
> config_vector = vf->nr_vring

I meant something like

if (DEV_SHARED)
    return 0;
else if  (SHARED_VQ_AND_CONFIG)
    config_vector = 1
else if (PER_VQ_AND_CONFIG)
    config_vector = vf->nr_vring
else
    return -EINVAL;

or using a switch here.

(We get the warning because there's no way for the checker to know
that msix_vector_status must be DEV_SHARED, SHARED_VQ_AND_CONFIG and
PER_VQ_AND_CONFIG).

Thanks

>
> Thanks,
> Zhu Lingshan
>
> >
> > Thanks
> >
> >
>