Re: [PATCH for-4.14 v2] x86/rtc: provide mediated access to RTC for PVH dom0

[Jan Beulich <jbeulich@xxxxxxxx>]

On 06.06.2020 01:43, Roman Shaposhnik wrote:
> On Fri, Jun 5, 2020 at 4:03 AM Roger Pau Monne <roger.pau@xxxxxxxxxx> wrote:
>>
>> Mediated access to the RTC was provided for PVHv1 dom0 using the PV
>> code paths (guest_io_{write/read}), but those accesses where never
>> implemented for PVHv2 dom0. This patch provides such mediated accesses
>> to the RTC for PVH dom0, just like it's provided for a classic PV
>> dom0.
>>
>> Pull out some of the RTC logic from guest_io_{read/write} into
>> specific helpers that can be used by both PV and HVM guests. The
>> setup of the handlers for PVH is done in rtc_init, which is already
>> used to initialize the fully emulated RTC.
>>
>> Without this a Linux PVH dom0 will read garbage when trying to access
>> the RTC, and one vCPU will be constantly looping in
>> rtc_timer_do_work.
>>
>> Note that such issue doesn't happen on domUs because the ACPI
>> NO_CMOS_RTC flag is set in FADT, which prevents the OS from accessing
>> the RTC. Also the X86_EMU_RTC flag is not set for PVH dom0, as the
>> accesses are not emulated but rather forwarded to the physical
>> hardware.
>>
>> No functional change expected for classic PV dom0.
> 
> For the dense guys like me: what is the user-visible feature that is now being
> offered with this? Would really appreciate a pointer or two.

PV Dom0 has always been permitted direct access to the hardware RTC.
This change makes PVH Dom0 follow suit.

Jan