Re: TSC in nested SVM and VMX

["Nadav Har'El" <nyh@math.technion.ac.il>]

On Sun, Oct 03, 2010, Alexander Graf wrote about "Re: TSC in nested SVM and VMX":
> Looking through the spec, the only indicator I've found is this passage:
> 
> TSC_OFFSET - an offset to add when the guest reads the TSC (time stamp
> counter). Guest writes to the TSC can be intercepted and emulated by
> changing the offset (without writing the physical TSC). This offset is
> cleared when the guest exits back to the host.
> 
> So apparently writes to TSC don't affect tsc_offset, but instead affect
> the host's tsc skew. So with nesting a non-intercepted tsc write affects
> L1's tsc_offset. This means the code is correct. Sorry for the fuss :).

I don't understand, how does this passage imply that writes to the TSC don't
affect the tsc_offset? It says that "writes to the TSC" can (I don't know why
this word was used...) "changing the offset". I don't understand why a guest
should be allowed to ruin its host's TSC (or in the nested case, why an L2
should be allowed to ruin L1's TSC without L1's knowledge) - isn't this
exactly why the TSC offset exists?

-- 
Nadav Har'El                        |      Sunday, Oct  3 2010, 25 Tishri 5771
nyh@math.technion.ac.il             |-----------------------------------------
Phone +972-523-790466, ICQ 13349191 |"Computers are useless. They can only
http://nadav.harel.org.il           |give you answers." -- Pablo Picasso