From: Boris Ostrovsky <boris.ostrovsky@oracle.com>
To: Jan Beulich <jbeulich@suse.com>,
"xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>
Cc: Juergen Gross <jgross@suse.com>, Konrad Wilk <konrad.wilk@oracle.com>
Subject: Re: [PATCH 2/3] xen-pciback: reconfigure also from backend watch handler
Date: Fri, 9 Apr 2021 17:43:09 -0400 [thread overview]
Message-ID: <a93c66e5-807b-e557-d437-48d7f46f25f7@oracle.com> (raw)
In-Reply-To: <74955b48-80b1-3436-06b4-d8569260aa65@suse.com>
On 4/7/21 10:37 AM, Jan Beulich wrote:
> When multiple PCI devices get assigned to a guest right at boot, libxl
> incrementally populates the backend tree. The writes for the first of
> the devices trigger the backend watch. In turn xen_pcibk_setup_backend()
> will set the XenBus state to Initialised, at which point no further
> reconfigures would happen unless a device got hotplugged. Arrange for
> reconfigure to also get triggered from the backend watch handler.
>
> Signed-off-by: Jan Beulich <jbeulich@suse.com>
> Cc: stable@vger.kernel.org
> ---
> I will admit that this isn't entirely race-free (with the guest actually
> attaching in parallel), but from the looks of it such a race ought to be
> benign (not the least thanks to the mutex). Ideally the tool stack
> wouldn't write num_devs until all devices had their information
> populated. I tried doing so in libxl, but xen_pcibk_setup_backend()
> calling xenbus_dev_fatal() when not being able to read that node
> prohibits such an approach (or else libxl and driver changes would need
> to go into use in lock-step).
>
> I wonder why what is being watched isn't just the num_devs node. Right
> now the watch triggers quite frequently without anything relevant
> actually having changed (I suppose in at least some cases in response
> to writes by the backend itself).
>
> --- a/drivers/xen/xen-pciback/xenbus.c
> +++ b/drivers/xen/xen-pciback/xenbus.c
> @@ -359,7 +359,8 @@ out:
> return err;
> }
>
> -static int xen_pcibk_reconfigure(struct xen_pcibk_device *pdev)
> +static int xen_pcibk_reconfigure(struct xen_pcibk_device *pdev,
> + enum xenbus_state state)
> {
> int err = 0;
> int num_devs;
> @@ -374,8 +375,7 @@ static int xen_pcibk_reconfigure(struct
>
> mutex_lock(&pdev->dev_lock);
> /* Make sure we only reconfigure once */
Is this comment still valid or should it be moved ...
> - if (xenbus_read_driver_state(pdev->xdev->nodename) !=
> - XenbusStateReconfiguring)
> + if (xenbus_read_driver_state(pdev->xdev->nodename) != state)
> goto out;
>
> err = xenbus_scanf(XBT_NIL, pdev->xdev->nodename, "num_devs", "%d",
> @@ -500,6 +500,9 @@ static int xen_pcibk_reconfigure(struct
> }
> }
>
> + if (state != XenbusStateReconfiguring)
> + goto out;
> +
... here?
> err = xenbus_switch_state(pdev->xdev, XenbusStateReconfigured);
> if (err) {
> xenbus_dev_fatal(pdev->xdev, err,
> @@ -525,7 +528,7 @@ static void xen_pcibk_frontend_changed(s
> break;
>
> case XenbusStateReconfiguring:
> - xen_pcibk_reconfigure(pdev);
> + xen_pcibk_reconfigure(pdev, XenbusStateReconfiguring);
> break;
>
> case XenbusStateConnected:
> @@ -664,6 +667,10 @@ static void xen_pcibk_be_watch(struct xe
> xen_pcibk_setup_backend(pdev);
> break;
>
> + case XenbusStateInitialised:
> + xen_pcibk_reconfigure(pdev, XenbusStateInitialised);
Could you add a comment here that this is needed when multiple devices are added?
It also looks a bit odd that adding a device is now viewed as a reconfiguration. It seems to me that xen_pcibk_setup_backend() and xen_pcibk_reconfigure() really should be merged --- initialization code for both looks pretty much the same.
-boris
next prev parent reply other threads:[~2021-04-09 21:43 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-07 14:35 [PATCH 0/3] xen-pciback: a fix, a workaround, and some simplification Jan Beulich
2021-04-07 14:37 ` [PATCH 1/3] xen-pciback: redo VF placement in the virtual topology Jan Beulich
2021-04-08 22:28 ` Boris Ostrovsky
2021-04-09 8:16 ` Jan Beulich
2021-04-07 14:37 ` [PATCH 2/3] xen-pciback: reconfigure also from backend watch handler Jan Beulich
2021-04-09 21:43 ` Boris Ostrovsky [this message]
2021-04-12 9:44 ` Jan Beulich
2021-04-12 15:55 ` Boris Ostrovsky
2021-04-07 14:37 ` [PATCH 3/3] xen-pciback: simplify vpci's find hook Jan Beulich
2021-04-09 21:45 ` Boris Ostrovsky
2021-04-12 9:50 ` Jan Beulich
2021-04-12 16:05 ` Boris Ostrovsky
2021-04-13 8:09 ` Jan Beulich
2021-04-13 12:54 ` Boris Ostrovsky
2021-04-23 8:05 ` Juergen Gross
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