Re: [PATCH v3 16/17] sbsa-gwdt: reschedule timer on direct WCV load

[Eric Auger <eauger@redhat.com>]

On 6/24/26 12:28 PM, Igor Mammedov wrote:
> According to spec[1]:
> 
>  "The compare value can either be loaded directly or indirectly on an
>   explicit refresh or timeout refresh"
> 
> QEMU accepts writes to WCV but doesn't reschedule the timer,
> which it should per the spec pseudo code:
> 
>  "TimeoutRefresh = ( SystemCounter [63:0] > CompareValue [63:0])"
> 
> Fix it by updating the timer on WCV write.
> 
> Fixes Windows in GTDT mode, which never issues a WRR refresh.
> Instead, it programs WOR to ~4 sec, enables WCS, and immediately
> writes a large absolute value into WCV to push the timeout far
> into the future. Without this fix, QEMU ignores the WCV write,
> the short WOR expires, and the guest reboots unexpectedly.
> 
> 1) Arm® Server Base System Architecture 6.0
>    Platform Design Document
>    DEN0029D 6.0
>    "A.2 Watchdog Operation"
> 
> Signed-off-by: Igor Mammedov <imammedo@redhat.com>
> ---
>  hw/watchdog/sbsa_gwdt.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/hw/watchdog/sbsa_gwdt.c b/hw/watchdog/sbsa_gwdt.c
> index 5ee0e06af5..fb67db9672 100644
> --- a/hw/watchdog/sbsa_gwdt.c
> +++ b/hw/watchdog/sbsa_gwdt.c
> @@ -44,6 +44,7 @@ static const VMStateDescription vmstate_sbsa_gwdt = {
>  typedef enum WdtRefreshType {
>      EXPLICIT_REFRESH = 0,
>      TIMEOUT_REFRESH = 1,
> +    WCV_LOAD = 2,
>  } WdtRefreshType;
>  
>  static uint64_t sbsa_gwdt_rread(void *opaque, hwaddr addr, unsigned int size)
> @@ -167,10 +168,16 @@ static void sbsa_gwdt_write(void *opaque, hwaddr offset, uint64_t data,
>  
>      case SBSA_GWDT_WCV:
>          s->wcvl = data;
> +        if (s->wcs & SBSA_GWDT_WCS_EN) {
> +            sbsa_gwdt_update_timer(s, WCV_LOAD);
regarding my comment on previous patch, you could add a WCV_LOAD check
in sbsa_gwdt_update_timer now.

In the previous patch it is not obvious that the code is identical to
the original code in default path (hence my previous comment)

Eric
> +        }
>          break;
>  
>      case SBSA_GWDT_WCVU:
>          s->wcvu = data;
> +        if (s->wcs & SBSA_GWDT_WCS_EN) {
> +            sbsa_gwdt_update_timer(s, WCV_LOAD);
> +        }
>          break;
>  
>      default: