linux-kernel - Re: [PATCH 2/2] xen-pciback: prepare for the split for stub and PV

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Date:   Thu, 23 Sep 2021 08:48:43 -0700 (PDT)
From:   Stefano Stabellini <sstabellini@...nel.org>
To:     Oleksandr Andrushchenko <Oleksandr_Andrushchenko@...m.com>
cc:     Juergen Gross <jgross@...e.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        "xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "boris.ostrovsky@...cle.com" <boris.ostrovsky@...cle.com>,
        "julien@....org" <julien@....org>,
        "jbeulich@...e.com" <jbeulich@...e.com>,
        Oleksandr Andrushchenko <andr2000@...il.com>
Subject: Re: [PATCH 2/2] xen-pciback: prepare for the split for stub and PV

On Thu, 23 Sep 2021, Oleksandr Andrushchenko wrote:
> On 23.09.21 12:05, Juergen Gross wrote:
> > On 23.09.21 11:02, Oleksandr Andrushchenko wrote:
> >>
> >> On 23.09.21 00:10, Stefano Stabellini wrote:
> >>> On Wed, 22 Sep 2021, Oleksandr Andrushchenko wrote:
> >>>> --- a/drivers/xen/xen-pciback/xenbus.c
> >>>> +++ b/drivers/xen/xen-pciback/xenbus.c
> >>>> @@ -743,6 +743,9 @@ const struct xen_pcibk_backend *__read_mostly xen_pcibk_backend;
> >>>>       int __init xen_pcibk_xenbus_register(void)
> >>>>    {
> >>>> +    if (!xen_pcibk_pv_support())
> >>>> +        return 0;
> >>> Is this truly enough to stop the PV backend from initializing? Have you
> >>> actually tested it to make sure? If it works, amazing! I am quite happy
> >>> about this approach :-)
> >>
> >> Well, I put some logs into the driver and saw nothing obvious pointing
> >>
> >> to any backend activities (probably this is also because I don't have any
> >>
> >> frontend). I see that the xenbus driver is not registered. In XenStore I see:
> >>
> >> root@...0:~# xenstore-ls -f | grep pci
> >> /local/domain/0/backend/pci = ""
> >> /local/domain/0/backend/pci/2 = ""
> >> /local/domain/0/backend/pci/2/0 = ""
> >> /local/domain/0/backend/pci/2/0/frontend = "/local/domain/2/device/pci/0"
> >> /local/domain/0/backend/pci/2/0/frontend-id = "2"
> >> /local/domain/0/backend/pci/2/0/online = "1"
> >> /local/domain/0/backend/pci/2/0/state = "1"
> >> /local/domain/0/backend/pci/2/0/domain = "DomU"
> >> /local/domain/0/backend/pci/2/0/key-0 = "0000:03:00.0"
> >> /local/domain/0/backend/pci/2/0/dev-0 = "0000:03:00.0"
> >> /local/domain/0/backend/pci/2/0/opts-0 = "msitranslate=0,power_mgmt=0,permissive=0,rdm_policy=strict"
> >> /local/domain/0/backend/pci/2/0/state-0 = "1"
> >> /local/domain/0/backend/pci/2/0/num_devs = "1"
> >> /local/domain/2/device/pci = ""
> >> /local/domain/2/device/pci/0 = ""
> >> /local/domain/2/device/pci/0/backend = "/local/domain/0/backend/pci/2/0"
> >> /local/domain/2/device/pci/0/backend-id = "0"
> >> /local/domain/2/device/pci/0/state = "1"
> >> /libxl/pci = ""
> >> /libxl/pci/0000-03-00-0 = ""
> >> /libxl/pci/0000-03-00-0/domid = "2"
> >>
> >> But IIUIC these come from the toolstack
> >>
> >> @Juergen, do you know how to check if the backend is indeed not running
> >>
> >> or the above should be enough to prove?
> >
> > I don't see how the backend could be running without being registered
> > with xenbus. It won't receive any watches, so there is no way a
> > connection with a frontend could be established.
> 
> This is my understanding too, so the only change I've put in patch I removed
> 
> register/unregister. It seems this is just enough and the patch should be ok as is

Fantastic! Thanks for checking.