Re: [PATCH kvmtool 1/6] rtc: Initialize the Register D for MC146818 RTC

[Andre Przywara <andre.przywara@arm.com>]

On Tue,  4 Dec 2018 11:14:28 +0000
Julien Thierry <julien.thierry@arm.com> wrote:

> From: Sami Mujawar <sami.mujawar@arm.com>
> 
> Some software drivers check the VRT bit (BIT7) of Register D before
> using the MC146818 RTC. Initialized the VRT bit in rtc__init() to
> indicate that the RAM and time contents are valid.
> 
> Signed-off-by: Sami Mujawar <sami.mujawar@arm.com>
> Signed-off-by: Julien Thierry <julien.thierry@arm.com>

Checked against the data sheet.

Reviewed-by: Andre Przywara <andre.przywara@arm.com>

This is quite interesting: we build the RTC emulation unconditionally
for every architecture, but don't expose it in the DT (for arm/arm64).
The Linux driver can't even be configured for arm64.
Interestingly it works if one pokes 0x70 and 0x71 directly in memory
from a guest. Which sounds hackish (do we want that?), but fits more a
less the firmware use case. We would just need to make sure it actually
works correctly on ARM, since nobody tested this properly before.

I guess EDK2 would just hardcode the address?

Cheers,
Andre.

> ---
>  hw/rtc.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/hw/rtc.c b/hw/rtc.c
> index 0649b5d..c1fa72f 100644
> --- a/hw/rtc.c
> +++ b/hw/rtc.c
> @@ -25,6 +25,11 @@
>  #define RTC_REG_C			0x0C
>  #define RTC_REG_D			0x0D
>  
> +/*
> + * Register D Bits
> + */
> +#define RTC_REG_D_VRT			(1 << 7)
> +
>  struct rtc_device {
>  	u8			cmos_idx;
>  	u8			cmos_data[128];
> @@ -140,6 +145,9 @@ int rtc__init(struct kvm *kvm)
>  		return r;
>  	}
>  
> +	/* Set the VRT bit in Register D to indicate valid RAM and
> time */
> +	rtc.cmos_data[RTC_REG_D] = RTC_REG_D_VRT;
> +
>  	return r;
>  }
>  dev_init(rtc__init);