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.