Re: [PATCH] x86/msr: fix handling of MSR_IA32_PERF_{STATUS/CTL}

["Roger Pau Monné" <roger.pau@citrix.com>]

Ping?

On Thu, Oct 15, 2020 at 03:34:12PM +0200, Roger Pau Monné wrote:
> On Wed, Oct 07, 2020 at 06:41:17PM +0200, Roger Pau Monné wrote:
> > On Wed, Oct 07, 2020 at 01:06:08PM +0100, Andrew Cooper wrote:
> > > On 06/10/2020 17:23, Roger Pau Monne wrote:
> > > > Currently a PV hardware domain can also be given control over the CPU
> > > > frequency, and such guest is allowed to write to MSR_IA32_PERF_CTL.
> > > 
> > > This might be how the current logic "works", but its straight up broken.
> > > 
> > > PERF_CTL is thread scope, so unless dom0 is identity pinned and has one
> > > vcpu for every pcpu, it cannot use the interface correctly.
> > 
> > Selecting cpufreq=dom0-kernel will force vCPU pinning. I'm not able
> > however to see anywhere that would force dom0 vCPUs == pCPUs.
> > 
> > > > However since commit 322ec7c89f6 the default behavior has been changed
> > > > to reject accesses to not explicitly handled MSRs, preventing PV
> > > > guests that manage CPU frequency from reading
> > > > MSR_IA32_PERF_{STATUS/CTL}.
> > > >
> > > > Additionally some HVM guests (Windows at least) will attempt to read
> > > > MSR_IA32_PERF_CTL and will panic if given back a #GP fault:
> > > >
> > > > vmx.c:3035:d8v0 RDMSR 0x00000199 unimplemented
> > > > d8v0 VIRIDIAN CRASH: 3b c0000096 fffff806871c1651 ffffda0253683720 0
> > > >
> > > > Move the handling of MSR_IA32_PERF_{STATUS/CTL} to the common MSR
> > > > handling shared between HVM and PV guests, and add an explicit case
> > > > for reads to MSR_IA32_PERF_{STATUS/CTL}.
> > > 
> > > OTOH, PERF_CTL does have a seemingly architectural "please disable turbo
> > > for me" bit, which is supposed to be for calibration loops.  I wonder if
> > > anyone uses this, and whether we ought to honour it (probably not).
> > 
> > If we let guests play with this we would have to save/restore the
> > guest value on context switch. Unless there's a strong case for this,
> > I would say no.
> > 
> > > > diff --git a/xen/include/xen/sched.h b/xen/include/xen/sched.h
> > > > index d8ed83f869..41baa3b7a1 100644
> > > > --- a/xen/include/xen/sched.h
> > > > +++ b/xen/include/xen/sched.h
> > > > @@ -1069,6 +1069,12 @@ extern enum cpufreq_controller {
> > > >      FREQCTL_none, FREQCTL_dom0_kernel, FREQCTL_xen
> > > >  } cpufreq_controller;
> > > >  
> > > > +static inline bool is_cpufreq_controller(const struct domain *d)
> > > > +{
> > > > +    return ((cpufreq_controller == FREQCTL_dom0_kernel) &&
> > > > +            is_hardware_domain(d));
> > > 
> > > This won't compile on !CONFIG_X86, due to CONFIG_HAS_CPUFREQ
> > 
> > It does seem to build on Arm, because this is only used in x86 code:
> > 
> > https://gitlab.com/xen-project/people/royger/xen/-/jobs/778207412
> > 
> > The extern declaration of cpufreq_controller is just above, so if you
> > tried to use is_cpufreq_controller on Arm you would get a link time
> > error, otherwise it builds fine. The compiler removes the function on
> > Arm as it has the inline attribute and it's not used.
> > 
> > Alternatively I could look into moving cpufreq_controller (and
> > is_cpufreq_controller) out of sched.h into somewhere else, I haven't
> > looked at why it needs to live there.
> > 
> > > Honestly - I don't see any point to this code.  Its opt-in via the
> > > command line only, and doesn't provide adequate checks for enablement. 
> > > (It's not as if we're lacking complexity or moving parts when it comes
> > > to power/frequency management).
> > 
> > Right, I could do a pre-patch to remove this, but I also don't think
> > we should block this fix on removing FREQCTL_dom0_kernel, so I would
> > rather fix the regression and then remove the feature if we agree it
> > can be removed.
> 
> Can we get some consensus on what to do next?
> 
> I think I've provided replies to all the points above, and I'm not sure
> what do to next in order to proceed with this patch.
> 
> Thanks, Roger.
>