Re: [PATCH] clocksource: arch_timer: Fix code to use physical timers when requested

[Olof Johansson]

On Wed, Sep 10, 2014 at 8:55 AM, Mark Rutland <mark.rutland@xxxxxxx> wrote:
> On Wed, Sep 10, 2014 at 03:58:15PM +0100, Christopher Covington wrote:
>> On 09/05/2014 06:11 PM, Doug Anderson wrote:
>> > Mark,
>> >
>> > On Thu, Aug 28, 2014 at 2:35 AM, Mark Rutland <mark.rutland@xxxxxxx> wrote:
>> >> Not if you boot Linux at hyp, as we've recommended for this precise
>> >> reason. That doesn't fix other things like CNTFRQ if the secure
>> >> initialisation doesn't poke that, however.
>> >
>> > I'll freely admit that I'm out of my league and out of my comfort zone
>> > here, but...
>> >
>> > In the theory that firmware ought to be as minimal as possible
>> > (because it's hard to update and hard to keep in sync with kernel
>> > versions), it seems like firmware ought to start the kernel out in as
>> > permissive mode as it's willing to provide, right?
>> >
>> > If the kernel is started out as permissive as possible then it can do
>> > anything it needs to.  Future versions of the kernel can be
>> > implemented to do any way-cool things that they want to do without an
>> > update to firmware, right?  ...and current versions of the kernel can
>> > just shed permissions if they don't want them.
>> >
>> > ...so if I understand correctly, "Secure SVC" mode is more permissive
>> > than "Non Secure HYP" mode, right?  It looks to me as if we currently
>> > start the kernel in "Secure SVC" mode.  What do you think about the
>> > kernel detecting Secure SVC and then dropping down permission levels
>> > (to Non Secure HYP).  Once it did this, it could update things like
>> > the virtual offset and then transition down further into non-secure
>> > SVC mode.
>> >
>> > ...or maybe this has been discussed millions of times already and I'm
>> > just clueless.  ...or maybe this is just too hard for the kernel to do
>> > in a generic way?
>>
>> I think this is a great idea. When running on simulators, it would make (the
>> non-DTB parts of) the bootwrapper and QEMU's built-in bootloader unnecessary.
>
> That's not true in general as other secure initialization will still be
> necessary, and the extent and character of that initialization is going
> to be implementation specific.
>
> If you have the infrastructure necessary to load and boot an arbitrary
> kernel at the most highly privileged exception level, then you
> necessarily have all the plumbing in place for loading an updateable
> firmware into that level. That latter option is preferable.

It might be preferable by you since it makes the kernel simpler, but
for the rest of us who are busy shipping product, it means extra work.
Why would we have to write a brand new firmware layer for this? No
fricking way.

> The issue at hand is not whether Linux should be in charge of secure
> world state. The issue at hand is that firmware booted Linux at PL1
> without taking ownership of all PL2 state (CNTVOFF included). Were
> either the firmware or Linux to manage PL2 (configuring CNTVOFF with a
> consistent value on all CPUs) that problem would not exist.

No, the issue at hand is that Linux is now expecting PL2 resources to
always have been set up. That is a regression in the true sense. It
didn't use to be the case, and now it is.



-Olof
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