linux-kernel - Re: [PATCH 09/16] vdpa/mlx5: Allow creation/deletion of any given mr struct

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Message-ID: <04488d248f425936fa8714552ed8b27c644a3257.camel@nvidia.com>
Date:   Mon, 9 Oct 2023 07:06:28 +0000
From:   Dragos Tatulea <dtatulea@...dia.com>
To:     "jasowang@...hat.com" <jasowang@...hat.com>
CC:     "xuanzhuo@...ux.alibaba.com" <xuanzhuo@...ux.alibaba.com>,
        Parav Pandit <parav@...dia.com>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
        "eperezma@...hat.com" <eperezma@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "si-wei.liu@...cle.com" <si-wei.liu@...cle.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "mst@...hat.com" <mst@...hat.com>,
        Saeed Mahameed <saeedm@...dia.com>
Subject: Re: [PATCH 09/16] vdpa/mlx5: Allow creation/deletion of any given mr
 struct

On Mon, 2023-10-09 at 14:39 +0800, Jason Wang wrote:
> On Sun, Oct 8, 2023 at 8:05 PM Dragos Tatulea <dtatulea@...dia.com> wrote:
> > 
> > On Sun, 2023-10-08 at 12:25 +0800, Jason Wang wrote:
> > > On Tue, Sep 26, 2023 at 3:21 PM Dragos Tatulea <dtatulea@...dia.com>
> > > wrote:
> > > > 
> > > > On Tue, 2023-09-26 at 12:44 +0800, Jason Wang wrote:
> > > > > On Tue, Sep 12, 2023 at 9:02 PM Dragos Tatulea <dtatulea@...dia.com>
> > > > > wrote:
> > > > > > 
> > > > > > This patch adapts the mr creation/deletion code to be able to work
> > > > > > with
> > > > > > any given mr struct pointer. All the APIs are adapted to take an
> > > > > > extra
> > > > > > parameter for the mr.
> > > > > > 
> > > > > > mlx5_vdpa_create/delete_mr doesn't need a ASID parameter anymore.
> > > > > > The
> > > > > > check is done in the caller instead (mlx5_set_map).
> > > > > > 
> > > > > > This change is needed for a followup patch which will introduce an
> > > > > > additional mr for the vq descriptor data.
> > > > > > 
> > > > > > Signed-off-by: Dragos Tatulea <dtatulea@...dia.com>
> > > > > > ---
> > > > > 
> > > > > Thinking of this decoupling I think I have a question.
> > > > > 
> > > > > We advertise 2 address spaces and 2 groups. So we actually don't know
> > > > > for example which address spaces will be used by dvq.
> > > > > 
> > > > > And actually we allow the user space to do something like
> > > > > 
> > > > > set_group_asid(dvq_group, 0)
> > > > > set_map(0)
> > > > > set_group_asid(dvq_group, 1)
> > > > > set_map(1)
> > > > > 
> > > > > I wonder if the decoupling like this patch can work and why.
> > > > > 
> > > > This scenario could indeed work. Especially if you look at the 13'th
> > > > patch
> > > > [0]
> > > > where hw support is added. Are you wondering if this should work at all
> > > > or
> > > > if it
> > > > should be blocked?
> > > 
> > > It would be great if it can work with the following patches. But at
> > > least for this patch, it seems not:
> > > 
> > > For example, what happens if we switch back to group 0 for dvq?
> > > 
> > > set_group_asid(dvq_group, 0)
> > > set_map(0)
> > > set_group_asid(dvq_group, 1)
> > > set_map(1)
> > > // here we destroy the mr created for asid 0
> > > set_group_asid(dvq_group, 0)
> > > 
> > If by destroy you mean .reset,
> 
> It's not rest. During the second map, the mr is destroyed by
> mlx5_vdpa_change_map().
> 
Oh, now I understand what you mean. This is not the case anymore. This patch
series introduces one mr per asid. mlx5_vdpa_change_map will only destroy the
old mr in the given asid. Before, there was one mr for all asids.

>  I think it works: During .reset the mapping in
> > ASID 0 is reset back to the DMA/pysical map (mlx5_vdpa_create_dma_mr). Am I
> > missing something?
> > 
> > > Btw, if this is a new issue, I haven't checked whether or not it
> > > exists before this series (if yes, we can fix on top).
> > 
> > > > 
> > > > > It looks to me the most easy way is to let each AS be backed by an MR.
> > > > > Then we don't even need to care about the dvq, cvq.
> > > > That's what this patch series dowes.
> > > 
> > > Good to know this, I will review the series.
> > > 
> > I was planning to spin a v3 with Eugenio's suggestions. Should I wait for
> > your
> > feedback before doing that?
> 
> You can post v3 and we can move the discussion there if you wish.
> 
Ack.

Thanks,
Dragos