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

[Wei Liu <wl@xxxxxxx>]

On Fri, Jun 05, 2020 at 01:02:39PM +0200, Roger Pau Monne 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.
> 
> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

Reviewed-by: Wei Liu <wl@xxxxxxx>