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

From: Geert Uytterhoeven <geert@linux-m68k.org>
To: Auger Eric <eric.auger@redhat.com>
Cc: Geert Uytterhoeven <geert+renesas@glider.be>,
	Baptiste Reynal <b.reynal@virtualopensystems.com>,
	Alex Williamson <alex.williamson@redhat.com>,
	"Rafael J . Wysocki" <rjw@rjwysocki.net>,
	Kevin Hilman <khilman@kernel.org>,
	Ulf Hansson <ulf.hansson@linaro.org>,
	KVM list <kvm@vger.kernel.org>,
	Linux PM list <linux-pm@vger.kernel.org>,
	Linux-Renesas <linux-renesas-soc@vger.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] vfio: platform: Fix using devices in PM Domains
Date: Fri, 13 Apr 2018 14:00:04 +0200	[thread overview]
Message-ID: <CAMuHMdVqX2efW=heQeRUG=iXWaROAtkNwy6VAOBikV+e+fcWLw@mail.gmail.com> (raw)
In-Reply-To: <4eedb2df-4bf8-814e-7e47-60faa646f4bf@redhat.com>

Hi Eric,

On Fri, Apr 13, 2018 at 11:44 AM, Auger Eric <eric.auger@redhat.com> wrote:
> On 13/04/18 11:19, Geert Uytterhoeven wrote:
>> On Fri, Apr 13, 2018 at 11:14 AM, Auger Eric <eric.auger@redhat.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@linux-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