linux-kernel - Re: [PATCH v2] vdpa_sim: set last_used_idx as last_avail_idx in vdpasim_queue

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Message-ID: <CAJaqyWcNO+McdRaHwe4ZV=jo-0n1AVgiRdNFh5J2Hq20qMJr1g@mail.gmail.com>
Date:   Thu, 2 Mar 2023 19:19:20 +0100
From:   Eugenio Perez Martin <eperezma@...hat.com>
To:     Stefano Garzarella <sgarzare@...hat.com>
Cc:     mst@...hat.com, lulu@...hat.com,
        virtualization@...ts.linux-foundation.org, si-wei.liu@...cle.com,
        leiyang@...hat.com, Gautam Dawar <gdawar@...inx.com>,
        Eli Cohen <elic@...dia.com>, longpeng2@...wei.com,
        parav@...dia.com, linux-kernel@...r.kernel.org,
        Zhu Lingshan <lingshan.zhu@...el.com>,
        alvaro.karsz@...id-run.com, Laurent Vivier <lvivier@...hat.com>
Subject: Re: [PATCH v2] vdpa_sim: set last_used_idx as last_avail_idx in vdpasim_queue_ready

On Tue, Feb 28, 2023 at 11:56 AM Stefano Garzarella <sgarzare@...hat.com> wrote:
>
> On Fri, Feb 03, 2023 at 03:25:01PM +0100, Eugenio Pérez wrote:
> >Starting from an used_idx different than 0 is needed in use cases like
> >virtual machine migration.  Not doing so and letting the caller set an
> >avail idx different than 0 causes destination device to try to use old
> >buffers that source driver already recover and are not available
> >anymore.
> >
> >Since vdpa_sim does not support receive inflight descriptors as a
> >destination of a migration, let's set both avail_idx and used_idx the
> >same at vq start.  This is how vhost-user works in a
> >VHOST_SET_VRING_BASE call.
> >
> >Although the simple fix is to set last_used_idx at vdpasim_set_vq_state,
> >it would be reset at vdpasim_queue_ready.  The last_avail_idx case is
> >fixed with commit a09f493c ("vdpa_sim: not reset state in
> >vdpasim_queue_ready").  Since the only option is to make it equal to
> >last_avail_idx, adding the only change needed here.
> >
> >This was discovered and tested live migrating the vdpa_sim_net device.
> >
> >Fixes: 2c53d0f64c06 ("vdpasim: vDPA device simulator")
> >Signed-off-by: Eugenio Pérez <eperezma@...hat.com>
> >---
> >Cherry-picked from patch 2/2 of the series [1]. Differences are:
> >* Set the value of used_idx at vdpasim_queue_ready instead of fetching
> >  from the guest vring like vhost-kernel.
> >
> >v2: Actually update last_used_idx only at vdpasim_queue_ready.
> >
> >Note that commit id present in the patch text is not in master but in
> >git.kernel.org/pub/scm/linux/kernel/git/mst/vhost.git.
> >
> >[1] https://lkml.org/lkml/2023/1/18/1041
> >---
> > drivers/vdpa/vdpa_sim/vdpa_sim.c | 1 +
> > 1 file changed, 1 insertion(+)
> >
> >diff --git a/drivers/vdpa/vdpa_sim/vdpa_sim.c b/drivers/vdpa/vdpa_sim/vdpa_sim.c
> >index 6a0a65814626..79ac585e40b9 100644
> >--- a/drivers/vdpa/vdpa_sim/vdpa_sim.c
> >+++ b/drivers/vdpa/vdpa_sim/vdpa_sim.c
> >@@ -68,6 +68,7 @@ static void vdpasim_queue_ready(struct vdpasim *vdpasim, unsigned int idx)
> >                         (uintptr_t)vq->device_addr);
> >
> >       vq->vring.last_avail_idx = last_avail_idx;
> >+      vq->vring.last_used_idx = last_avail_idx;
> >       vq->vring.notify = vdpasim_vq_notify;
> > }
> >
> >-- 2.31.1
> >
>
> If you need to resend, I'd add a comment in the code following the
> commit description.
>
> Anyway, the patch LGTM:
>
> Reviewed-by: Stefano Garzarella <sgarzare@...hat.com>
>

Added in v3, thanks for the review!

> Thanks,
> Stefano
>