Re: [Xen-devel] Xen ARM - Exposing a PL011 to the guest

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Wed, 15 Feb 2017, Bhupinder Thakur wrote:
> On 15 February 2017 at 13:45, Bhupinder Thakur
> <bhupinder.thakur@xxxxxxxxxx> wrote:
> > Hi Stefano,
> >
> > On 14 February 2017 at 03:14, Stefano Stabellini <sstabellini@xxxxxxxxxx> 
> > wrote:
> >> On Mon, 13 Feb 2017, Bhupinder Thakur wrote:
> >>> Hi Stefano,
> >>>
> >>> On 9 February 2017 at 05:40, Stefano Stabellini <sstabellini@xxxxxxxxxx> 
> >>> wrote:
> >>> > On Wed, 8 Feb 2017, Bhupinder Thakur wrote:
> >>> >> Hi Julien,
> >>> >>
> >>> >> On 3 February 2017 at 19:38, Julien Grall <julien.grall@xxxxxxx> wrote:
> >>> >> > So if I understand correctly, you don't receive anymore output. 
> >>> >> > Correct?
> >>> >> > Have you tried to see whether the pl011 driver is receiving 
> >>> >> > interrupt or
> >>> >> > even Linux calling it.
> >>> >>
> >>> >> The pl011 driver is not receiving the interrupts (I put a
> >>> >> HYPERVISOR_console_io() call inside the interrupt handler and I do not
> >>> >> see the print on the xen console) . When the console is in this state,
> >>> >> if I keep typing the characters on the console then Xen keeps raising
> >>> >> the interrupts (for each character typed) but all those interrupts are
> >>> >> marked as inflight. So it seems that interrupt is not getting cleared
> >>> >> in GIC. Which GIC function can I look at to further debug what is the
> >>> >> issue?
> >>> >>
> >>> >> Earlier, when I used to manually write characters to /dev/ttyAMA0 then
> >>> >> I used to receive the interrupts all fine.
> >>> >
> >>> > DomU doesn't get emulated PL011 interrupts from Xen? Make sure you don't
> >>> > mark the interrupt as a real hardware interrupt by mistake
> >>> > (GICH_V2_LR_HW). At the same time, check that the virtual PL011
> >>> > interrupt has been added correctly to the LRs
> >>> > (xen/arch/arm/gic-v2.c:gicv2_update_lr).
> >>>
> >>> I verified that the PL011 interrupt is getting marked pending by Xen
> >>> but guest does not receive it. I am not sure if the local interrupt
> >>> delivery is disabled on the guest because of which it may not be
> >>> receiving the interrupts. Is there a way to verify that interrupts are
> >>> indeed disabled at that point. Since the interrupt delivery works fine
> >>> when I manually read/write characters to /dev/ttyAMA0, I think there
> >>> is no issue from the Xen side in terms of raising an interrupt.
> >>>
> >>> One observation is that pl011_request_port () calls request_mem_region
> >>> () for the MMIO space allocated to pl011. The function returns -EBUSY
> >>> indicating that the memory region is already in use. But when I do cat
> >>> /proc/iomem, I do not see any overlaps. I am using 0x22000000 as the
> >>> start of the pl011 MMIO space. I see the same error while booting the
> >>> dom0 kernel also.
> >>>
> >>> I have attached the guest boot logs when console=ttyAMA0. I have added
> >>> debug prints marked with [DEBUG]. I added debug prints in amba-pl011.c
> >>> and serial_core.c. The output from the console seems to suddenly stop
> >>> once the pl011 port is started. I have not added getty for ttyAMA0 in
> >>> the /etc/inittab file. I believe auto-serial-console should be able to
> >>> attach a getty session to ttyAMA0 automatically (i did try with adding
> >>> a getty command in the inittab file but that did not solve the issue).
> >>>
> >>> Towards the end of the log, we can see that every time a character is
> >>> typed, Xen raises an interrupt and the characters keep getting
> >>> collected in the IN RING buffer (in_prod keeps incrementing). Later
> >>> interrupts remain in inflight as the earlier interrupt is not cleared.
> >>
> >> Let me get this straight: who is printing all those messages we see on
> >> the guest console (xl console guest) up until "Freeing unused kernel
> >> memory"?  Is it the early pl011 driver? Or is it the Xen PV console
> >> driver?
> >>
> >> Are you using a DEBUG build of Xen? Only DEBUG builds allow DomUs to
> >> issue HYPERVISOR_console_io hypercalls. You might want to check that
> >> do_console_io in Xen is letting DomU calls through.
> >>
> >> Otherwise pl011_int might actually be called but the printk might not
> >> show up on the Xen console.
> >>
> >> The interrupt is enabled from Xen point of view, otherwise it would not
> >> be added to the LR. However Linux might still refuse to call the
> >> interrupt handler for some reason. You might want to add a printk in the
> >> Linux generic interrupt handler, probably
> >> kernel/irq/chip.c:handle_fasteoi_irq in your case.
> >
> > The pl011 console is working now. I did not fix any thing specific
> > except I cleaned up the debug code which I had added. I am not sure if
> > some of my debug code was causing the issue.
> >
> > After cleaning up the debug code and building the code afresh, I
> > started receiving the interrupts and input/output on the console is
> > working fine. Now I am able to execute commands on the console.
> >
> I believe since I was using printk in the pl011 console driver code
> for debugging, that might be causing some infinite loop. Once I
> replaced all printks with HYPERVISOR_console_io(), it started working.
> Later I compiled out the debug code, and it was working fine.

Yep, I tripped over that problem myself at least once or twice :-)

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