On 11/09/14 18:11, Doug Anderson wrote: > Hi, > > On Thu, Sep 11, 2014 at 10:00 AM, Marc Zyngier <marc.zyngier@xxxxxxx> wrote: >> On 11/09/14 17:47, Will Deacon wrote: >>> On Thu, Sep 11, 2014 at 05:16:44PM +0100, Doug Anderson wrote: >>>> Some 32-bit (ARMv7) systems are architected like this: >>>> >>>> * The firmware doesn't know and doesn't care about hypervisor mode and >>>> we don't want to add the complexity of hypervisor there. >>>> >>>> * The firmware isn't involved in SMP bringup or resume. >>>> >>>> * The ARCH timer come up with an uninitialized offset between the >>>> virtual and physical counters. Each core gets a different random >>>> offset. >>>> >>>> On systems like the above, it doesn't make sense to use the virtual >>>> counter. There's nobody managing the offset and each time a core goes >>>> down and comes back up it will get reinitialized to some other random >>>> value. >>> >>> You probably need to rephrase this slightly, as there *is* still a >>> requirement on the hypervisor/firmware (actually, two!). See below. >>> >>>> Let's add a property to the device tree to say that we shouldn't use >>>> the virtual timer. Firmware could potentially remove this property >>>> before passing the device tree to the kernel if it really wants the >>>> kernel to use a virtual timer. >>>> >>>> Note that it's been said that ARM64 (ARMv8) systems the firmware and >>>> kernel really can't be architected as described above. That means >>>> using the physical timer like this really only makes sense for ARMv7 >>>> systems. >>> >>> I'd go further: this only makes sense if you're booting in secure SVC >>> mode. >> >> If that's the case, what's the problem? Enter monitor mode, set SCR.NS >> to one, nuke CNTVOFF, revert, job done. >> >> What am I missing? > > Stuff like this was talked about in the thread about Sonny's patch at > <https://patchwork.kernel.org/patch/4790921/> > > ...in that case we were always talking about HYP mode, though. I That's because I always assumed that you'd be running non-secure, dropped there by some idiotic firmware without any way to go back up. > don't think anyone has explicitly talked about just switching to > monitor mode and then leaving ourselves in Secure SVC after we're > done. It would be nice (especially for the VDSO guys) if we could > just init the virtual offset... > > We would need to run this code potentially at processor bringup and > after suspend/resume, but that seems possible too. Note that this would be an ARMv7 only thing (you can't do that on ARMv8, at all). > Is the transition to monitor mode and back simple? Where would you > suggest putting this code? It would definitely need to be pretty > early. We'd also need to be able to detect that we're in Secure SVC > and not mess up anyone else who happened to boot in Non Secure SVC. This would have to live in some very early platform-specific code. The ugly part is that you cannot find out what world you're in (accessing SCR is going to send you to UNDEF-land if accessed from NS). If I was suicidal, I'd suggest you could pass a parameter to the command line, interpreted by the timer code... But I since I'm not, let's pretend I haven't said anything... ;-) M. -- Jazz is not dead. It just smells funny... -- To unsubscribe from this list: send the line "unsubscribe devicetree" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
