linux-kernel - Re: [PATCH] vfio: platform: Fix using devices in PM Domains

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Date:   Fri, 13 Apr 2018 14:00:04 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Auger Eric <eric.auger@...hat.com>
Cc:     Geert Uytterhoeven <geert+renesas@...der.be>,
        Baptiste Reynal <b.reynal@...tualopensystems.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        "Rafael J . Wysocki" <rjw@...ysocki.net>,
        Kevin Hilman <khilman@...nel.org>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        KVM list <kvm@...r.kernel.org>,
        Linux PM list <linux-pm@...r.kernel.org>,
        Linux-Renesas <linux-renesas-soc@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] vfio: platform: Fix using devices in PM Domains

Hi Eric,

On Fri, Apr 13, 2018 at 11:44 AM, Auger Eric <eric.auger@...hat.com> wrote:
> On 13/04/18 11:19, Geert Uytterhoeven wrote:
>> On Fri, Apr 13, 2018 at 11:14 AM, Auger Eric <eric.auger@...hat.com> wrote:
>>> On 11/04/18 11:24, Geert Uytterhoeven wrote:
>>>> If a device is part of a PM Domain (e.g. power and/or clock domain), its
>>>> power state is managed using Runtime PM.  Without Runtime PM, the device
>>>> may not be powered up, causing subtle failures, crashes, or system
>>>> lock-ups when the device is accessed by the guest.
>>> the device may not be powered up/clcoked or power/clock may be switched
>>> off while the guest uses it.
>>>>
>>>> Fix this by adding Runtime PM support, powering the device when the VFIO
>>>> device is opened by the guest.
>>>>
>>>> Note that while more fine-grained power management could be implemented
>>>> on the guest side, if exported, this would be inherently unsafe, as
>>>> abusing it may kill the whole system.
>>>
>>> Please can you elaborate on this remark please?
>>
>> If power-management of the device would be delegated to the guest, and the
>> guest forgets to enable device power before accessing the device's registers,
>> this could lock up the system, and thus disturb both the host and other guests.
>
> Wouldn't you need to assign another device or use para-virt to allow the
> guest to perform this power management control? I think you can remove
> this paragraph from the commit message.

Yes, that needs additional exporting of power-management (cfr. the "if
exported").
I just wanted to mention the caveat, as eventually, someone is going to
suggest doing this ;-)

But if you think the comment should be removed, I can of course do so.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds