Re: [PATCH for-4.14 v3] x86/rtc: provide mediated access to RTC for PVH dom0

[Jan Beulich <jbeulich@xxxxxxxx>]

On 08.06.2020 13:45, Roger Pau Monné wrote:
> On Mon, Jun 08, 2020 at 12:12:40PM +0100, Paul Durrant wrote:
>>> From: Roger Pau Monne <roger.pau@xxxxxxxxxx>
>>> Sent: 08 June 2020 11:30
>>>
>>> @@ -1110,6 +1111,67 @@ static unsigned long get_cmos_time(void)
>>>      return mktime(rtc.year, rtc.mon, rtc.day, rtc.hour, rtc.min, rtc.sec);
>>>  }
>>>
>>> +/* Helpers for guest accesses to the physical RTC. */
>>> +unsigned int rtc_guest_read(unsigned int port)
>>> +{
>>> +    const struct domain *currd = current->domain;
>>> +    unsigned long flags;
>>> +    unsigned int data = ~0;
>>> +
>>> +    switch ( port )
>>> +    {
>>> +    case RTC_PORT(0):
>>> +        /*
>>> +         * All PV domains are allowed to read the latched value of the 
>>> first
>>> +         * RTC port. This is useful in order to store data when debugging.
>>> +         */
>>
>> Is this comment correct. AFAICT your call to register_portio_handler() would 
>> allow a PVH dom0 to access this too.
> 
> Oh, maybe this is not clear enough. Yes, PV/PVH dom0 will get access
> to both ports, but a PV domU will also get read/write access to the
> latched value in the first RTC port, even when it doesn't have access
> to the second RTC port.

I'd word this slightly differently: "..., even when it doesn't have
access to either physical RTC port."

Jan