Re: [PATCH v2] x86/amd: do not expose HWCR.TscFreqSel to guests

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Thu, Sep 14, 2023 at 03:16:40PM +0200, Jan Beulich wrote:
> On 14.09.2023 14:57, Roger Pau Monné wrote:
> > On Thu, Sep 14, 2023 at 02:49:45PM +0200, Jan Beulich wrote:
> >> On 14.09.2023 14:37, Roger Pau Monné wrote:
> >>> On Thu, Sep 14, 2023 at 07:52:33AM +0200, Jan Beulich wrote:
> >>>> On 13.09.2023 16:52, Roger Pau Monne wrote:
> >>>>> OpenBSD 7.3 will unconditionally access HWCR if the TSC is reported as
> >>>>> Invariant, and it will then attempt to also unconditionally access 
> >>>>> PSTATE0 if
> >>>>> HWCR.TscFreqSel is set (currently the case on Xen).
> >>>>>
> >>>>> The relation between HWCR.TscFreqSel and PSTATE0 is not clearly written 
> >>>>> down in
> >>>>> the PPR, but it's natural for OSes to attempt to fetch the P0 frequency 
> >>>>> if the
> >>>>> TSC increments at the P0 frequency.
> >>>>>
> >>>>> Exposing PSTATEn (PSTATE0 at least) with all zeroes is not a suitable 
> >>>>> solution
> >>>>> because the PstateEn bit is read-write, and OSes could legitimately 
> >>>>> attempt to
> >>>>> set PstateEn=1 which Xen couldn't handle.
> >>>>>
> >>>>> In order to fix expose an empty HWCR, which is an architectural MSR and 
> >>>>> so must
> >>>>> be accessible.
> >>>>>
> >>>>> Note it was not safe to expose the TscFreqSel bit because it is not
> >>>>> architectural, and could change meaning between models.
> >>>>
> >>>> This imo wants (or even needs) extending to address the aspect of then
> >>>> exposing, on newer families, a r/o bit with the wrong value.
> >>>
> >>> We could always be exposing bits with the wrong values on newer
> >>> (unreleased?) families, I'm not sure why it needs explicit mentioning
> >>> here.
> >>
> >> Hmm, yes, that's one way to look at things. Yet exposing plain zero is
> >> pretty clearly not within spec here,
> > 
> > As I understand it, the fact that HWCR.TscFreqSel is read-only doesn't
> > exclude the possibility of it changing using other means (iow: we
> > should consider that a write to a different register could have the
> > side effect of toggling the bit).
> > 
> > The PPR I'm reading doesn't mention that the bit must be 1, just that
> > it's 1 on reset and read-only.
> 
> Sure; the PPR being incomplete doesn't help here. My interpretation, based
> on the bit having been r/w in earlier families, is that AMD wanted to retain
> its meaning without allowing it to be configurable anymore. Possibly a sign
> of it remaining so going forward.

What about:

"Note it was not safe to expose the TscFreqSel bit because it is not
architectural, and could change meaning between models.  Since HWCR
contains both architectural and non-architectural bits, going forward
care must be taken to assert the exposed value is correct on newer
CPU families."

Thanks, Roger.