Re: [RFC PATCH] ARM: mach-shmobile: Parse DT to get ARCH timer memory region

[Geert Uytterhoeven]

Hi Oleksandr,

On Mon, May 13, 2019 at 4:22 PM Oleksandr <olekstysh@xxxxxxxxx> wrote:
> On 13.05.19 12:19, Julien Grall wrote:
> > On 5/10/19 5:22 PM, Oleksandr Tyshchenko wrote:
> >> From: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
> >>
> >> Don't use hardcoded address, retrieve it from device-tree instead.
> >>
> >> And besides, this patch fixes the memory error when running
> >> on top of Xen hypervisor:
> >>
> >> (XEN) traps.c:1999:d0v0 HSR=0x93830007 pc=0xc0b097f8 gva=0xf0805000
> >>        gpa=0x000000e6080000
> >>
> >> Which shows that VCPU0 in Dom0 is trying to access an address in memory
> >> it is not allowed to access (0x000000e6080000).
> >> Put simply, Xen doesn't know that it is a device's register memory
> >> since it wasn't described in a host device tree (which Xen parses)
> >> and as the result this memory region wasn't assigned to Dom0 at
> >> domain creation time.
> >>
> >> Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
> >>
> >> ---
> >>
> >> This patch is meant to get feedback from the community before
> >> proceeding further. If we decide to go this direction, all Gen2
> >> device-trees should be updated (add memory region) before
> >> this patch going in.
> >>
> >> e.g. r8a7790.dtsi:
> >>
> >> ...
> >> timer {
> >>     compatible = "arm,armv7-timer";
> >>     interrupts-extended = <&gic GIC_PPI 13 (GIC_CPU_MASK_SIMPLE(8) |
> >> IRQ_TYPE_LEVEL_LOW)>,
> >>                   <&gic GIC_PPI 14 (GIC_CPU_MASK_SIMPLE(8) |
> >> IRQ_TYPE_LEVEL_LOW)>,
> >>                   <&gic GIC_PPI 11 (GIC_CPU_MASK_SIMPLE(8) |
> >> IRQ_TYPE_LEVEL_LOW)>,
> >>                   <&gic GIC_PPI 10 (GIC_CPU_MASK_SIMPLE(8) |
> >> IRQ_TYPE_LEVEL_LOW)>;
> >> +     reg = <0 0xe6080000 0 0x1000>;
> >
> > This looks incorrect, the "arm,armv7-timer" bindings doesn't offer you
> > the possibility to specify an MMIO region. This makes sense because it
> > is meant to describe the Arch timer that is only access via
> > co-processor registers.
> >
> > Looking at the code, I think the MMIO region corresponds to the
> > coresight (based on the register name). So you may want to describe
> > the coresight in the Device-Tree.
> >
> > Also, AFAICT, the code is configuring and turning on the timer if it
> > has not been done yet. If you are here running on Xen, then you have
> > probably done something wrong. Indeed, it means Xen would not be able
> > to use the timer until Dom0 has booted. But, shouldn't newer U-boot do
> > it for you?
>
> Let me elaborate a bit more on this...
>
> Indeed, my PSCI patch series for U-Boot includes a patch [1] for
> configuring that "counter module". So, if PSCI is available
> (psci_smp_available() == true), then most likely we are running on
> PSCI-enabled
> U-Boot which, we assume, has already taken care of configuring timer (as
> well as resetting CNTVOFF). So, when running on Xen, the timer was
> configured beforehand in U-Boot, and Xen is able to use it from the very
> beginning, these is no need to wait for Dom0 to configure it.
>
> (XEN) Generic Timer IRQ: phys=30 hyp=26 virt=27 Freq: 10000 KHz
>
> So, the code in brackets won't be called when using PSCI/running Xen,
> since the timer is already both enabled and configured:
>
> if ((ioread32(base + CNTCR) & 1) == 0 ||
>          ioread32(base + CNTFID0) != freq) {
>          /* Update registers with correct frequency */
>          iowrite32(freq, base + CNTFID0);
>          asm volatile("mcr p15, 0, %0, c14, c0, 0" : : "r" (freq));
>
>          /* make sure arch timer is started by setting bit 0 of CNTCR */
>          iowrite32(1, base + CNTCR);
> }
>
> But, the problem here is the first read access from timer register (when
> we check whether the timer requires enabling) results in hypervisor trap:
>
> (XEN) traps.c:1999:d0v0 HSR=0x93830007 pc=0xc0b097f8 gva=0xf0805000
> gpa=0x000000e6080000
>
> So, if the DT bindings for the counter module is not an option (if I
> correctly understood a discussion pointed by Geert in another letter),
> we should probably prevent all timer code here from being executed if
> PSCI is in use.
> What I mean is to return to [2], but with the modification to use
> psci_smp_available() helper as an indicator of PSCI usage.
>
> Julien, Geert, what do you think?

Yes, that sounds good to me.

Note that psci_smp_available() seems to return false if CONFIG_SMP=n,
so checking for that is not sufficient to avoid crashes when running a
uniprocessor kernel on a PSCI-enabled system.


Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

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