Re: [Xen-devel] [Patch] x86/HVM: Fix RTC interrupt modelling

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 10.02.14 at 19:43, Andrew Cooper <andrew.cooper3@xxxxxxxxxx> wrote:
> REG_B and REG_C are to do with interrupts, and which events
> should(B)/have(C) generated interrupts.  The worst case is that a guest
> gets none/too-few/too-many interrupts when trying to drive the RTC. 
> None of this should lead to clock skew, as reading the time values
> directly will still provide the same information as before, although any
> guest which attempts to guess time based on counting periodic interrupts
> from the RTC is a) already broken

Why? If an OS itself guarantees low enough interrupt latency (for
the chosen period), no interrupt would ever get lost, and counting
would be precise afaict.

> and b) already having massive skew as
> a VM due to vcpu scheduling.

Yes, very likely. But that's a problem we (the ones virtualizing the
OS) have to solve, not the OS (it would be the other way around
only if the OS was PV or at least virtualization aware).

Jan


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