Re: [PATCH 08/11] arm: virt: create GWDT watchdog paired with WDAT ACPI table

["Daniel P. Berrangé" <berrange@redhat.com>]

On Thu, Feb 19, 2026 at 01:17:51PM +0100, Igor Mammedov wrote:
> On Wed, 18 Feb 2026 19:08:36 +0000
> Peter Maydell <peter.maydell@linaro.org> wrote:
> 
> > On Fri, 6 Feb 2026 at 13:15, Igor Mammedov <imammedo@redhat.com> wrote:
> > >
> > > Add SBSA generic watchdog to virt machine type with
> > > all necessary wiring for ACPI watchdog. Which includes
> > > setting its frequency to 1KHz (max that WDAT is able to handle).
> > >
> > > Signed-off-by: Igor Mammedov <imammedo@redhat.com>
> > > ---  
> > 
> > > diff --git a/hw/arm/virt.c b/hw/arm/virt.c
> > > index 390845c503..caf5700ed2 100644
> > > --- a/hw/arm/virt.c
> > > +++ b/hw/arm/virt.c
> > > @@ -93,6 +93,7 @@
> > >  #include "hw/cxl/cxl.h"
> > >  #include "hw/cxl/cxl_host.h"
> > >  #include "qemu/guest-random.h"
> > > +#include "hw/watchdog/sbsa_gwdt.h"
> > >
> > >  static GlobalProperty arm_virt_compat[] = {
> > >      { TYPE_VIRTIO_IOMMU_PCI, "aw-bits", "48" },
> > > @@ -194,6 +195,8 @@ static const MemMapEntry base_memmap[] = {
> > >      [VIRT_PVTIME] =             { 0x090a0000, 0x00010000 },
> > >      [VIRT_SECURE_GPIO] =        { 0x090b0000, 0x00001000 },
> > >      [VIRT_ACPI_PCIHP] =         { 0x090c0000, ACPI_PCIHP_SIZE },
> > > +    [VIRT_GWDT_REFRESH] =       { 0x090d0000, 0x00001000 },
> > > +    [VIRT_GWDT_CONTROL] =       { 0x090d1000, 0x00001000 },
> > >      [VIRT_MMIO] =               { 0x0a000000, 0x00000200 },
> > >      /* ...repeating for a total of NUM_VIRTIO_TRANSPORTS, each of that size */
> > >      [VIRT_PLATFORM_BUS] =       { 0x0c000000, 0x02000000 },
> > > @@ -245,12 +248,32 @@ static const int a15irqmap[] = {
> > >      [VIRT_GPIO] = 7,
> > >      [VIRT_UART1] = 8,
> > >      [VIRT_ACPI_GED] = 9,
> > > +    [VIRT_GWDT_WS0] = 10,
> > >      [VIRT_MMIO] = 16, /* ...to 16 + NUM_VIRTIO_TRANSPORTS - 1 */
> > >      [VIRT_GIC_V2M] = 48, /* ...to 48 + NUM_GICV2M_SPIS - 1 */
> > >      [VIRT_SMMU] = 74,    /* ...to 74 + NUM_SMMU_IRQS - 1 */
> > >      [VIRT_PLATFORM_BUS] = 112, /* ...to 112 + PLATFORM_BUS_NUM_IRQS -1 */
> > >  };
> > >
> > > +static void create_wdt(const VirtMachineState *vms)
> > > +{
> > > +    hwaddr rbase = vms->memmap[VIRT_GWDT_REFRESH].base;
> > > +    hwaddr cbase = vms->memmap[VIRT_GWDT_CONTROL].base;
> > > +    int irq = vms->irqmap[VIRT_GWDT_WS0];
> > > +    DeviceState *dev = qdev_new(TYPE_WDT_SBSA);
> > > +    SysBusDevice *s = SYS_BUS_DEVICE(dev);
> > > +
> > > +    /*
> > > +     * Set watchdog tick freq to 1Kz as it's the max WDAT driver
> > > +     * is able to handle.
> > > +     */
> > > +    qdev_prop_set_uint64(dev, "clock-frequency", 1000 /* 1KHz */);
> > > +    sysbus_realize_and_unref(s, &error_fatal);
> > > +    sysbus_mmio_map(s, 0, rbase);
> > > +    sysbus_mmio_map(s, 1, cbase);
> > > +    sysbus_connect_irq(s, 0, qdev_get_gpio_in(vms->gic, irq));
> > > +}  
> > 
> > Please can you also add support for exposing this device
> > in the device tree ?
> 
> It's possible,
> but we probably should not enable it if acpi variant was requested,
> to avoid confusion on guest side.
> 
> For Windows it doesn't really mater, for linux it does.
> on x86 linux guest uses a quirk to disable native iTCO watchdog
> in favor of WDAT one if later is present.
> I assume quirk is not desirable so we should expose only a preferred
> variant.
> 
> > 
> > > +
> > >  static void create_randomness(MachineState *ms, const char *node)
> > >  {
> > >      struct {
> > > @@ -2515,6 +2538,9 @@ static void machvirt_init(MachineState *machine)
> > >      vms->highmem_ecam &= (!firmware_loaded || aarch64);
> > >
> > >      create_rtc(vms);
> > > +    if (machine->acpi_watchdog) {
> > > +        create_wdt(vms);
> > > +    }  
> > 
> > Can we have a command line option name that isn't ACPI
> > specific, please? There's nothing inherent to ACPI about
> > "I would like a watchdog device".
> 
> that is specifically asking for ACPI flavor being used/configured.
> acpi specific option conflates 2 things:
>    1. watchdog device creation (of the board choice) with properties tuned for WDAT usage
>    2. how to expose it on firmware level (in this case ACPI WDAT table)
> 
> other option, I've considered was
>  -device some_wd[,fw=dt|acpi|none]
> but then 'fw' is not exactly device property but rather a machine one
> (we can 'abuse' it/use as a proxy of cause).
> However it doesn't work for boards that have builtin watchdog.

Right, if there was a case where we were already using -device to
create the watchdog, then I would suggest having a simple
'wadt=on|off' property as standard for any watchdog wanting that
ACPI abstraction wrapper.

That could (hypothetically) be the case if we had chosen to add
WADT suport backed by i6300esb, eg -device i6300esb,wadt=on|off

For cases built-in to the machine type, I'd suggest <type>-wadt=on|off

eg for Q35,  "-machine q35,itco-wadt=on|off"

for virt, '-machine virt,gwdt-wadt=on|off'

Q35 is easier as we rely on the guest quirks to disable direct
access via both paths.

With GWDT we need to decide if gwdt-wadt=on should imply DTB
disabled, or if we just expose both & rely on a guest quirk
or worst case, need a separate property to toggle DTB too.

> I've used device property for x86 Q35 TCO watchdog and
> then using -global to enable it as workaround in previous version.
> Ugly but doable workaround. It puts a burden on users to learn how
> to configure it for each support watchdog/board combo.
> It's nightmare to discover/maintain though.
> 
> Hence in this version, I've used a generic machine property approach
> that exposes feature in uniform way across boards.
> 
> enum might work, but ...
>   machine.watchdog = [acpi | what else could we put here???]
> 
> I'm not married to a way how we expose/configure it and will rewrite
> it to something that works for majority.
> 
> > thanks
> > -- PMM
> > 
> 

With regards,
Daniel
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