On Wed, 20 Oct 2021 at 12:25, Sudeep Holla <sudeep.holla@xxxxxxx> wrote: > > On Fri, Oct 15, 2021 at 11:17:18AM +0200, Ulf Hansson wrote: > > On Thu, 14 Oct 2021 at 16:56, Sudeep Holla <sudeep.holla@xxxxxxx> wrote: > > > > > > On Thu, Oct 14, 2021 at 12:56:46PM +0200, Ulf Hansson wrote: > > > > On Mon, 17 May 2021 at 18:14, Sudeep Holla <sudeep.holla@xxxxxxx> wrote: > > > > > > > > > > The CLKSCREW attack [0] exposed security vulnerabilities in energy management > > > > > implementations where untrusted software had direct access to clock and > > > > > voltage hardware controls. In this attack, the malicious software was able to > > > > > place the platform into unsafe overclocked or undervolted configurations. Such > > > > > configurations then enabled the injection of predictable faults to reveal > > > > > secrets. > > > > > > > > > > Many Arm-based systems used to or still use voltage regulator and clock > > > > > frameworks in the kernel. These frameworks allow callers to independently > > > > > manipulate frequency and voltage settings. Such implementations can render > > > > > systems susceptible to this form of attack. > > > > > > > > > > Attacks such as CLKSCREW are now being mitigated by not having direct and > > > > > independent control of clock and voltage in the kernel and moving that > > > > > control to a trusted entity, such as the SCP firmware or secure world > > > > > firmware/software which are to perform sanity checking on the requested > > > > > performance levels, thereby preventing any attempted malicious programming. > > > > > > > > > > With the advent of such an abstraction, there is a need to replace the > > > > > generic clock and regulator bindings used by such devices with a generic > > > > > performance domains bindings. > > > > > > > > > > [0] https://www.usenix.org/conference/usenixsecurity17/technical-sessions/presentation/tang > > > > > > > > > > Link: https://lore.kernel.org/r/20201116181356.804590-1-sudeep.holla@xxxxxxx > > > > > Cc: Rob Herring <robh+dt@xxxxxxxxxx> > > > > > Acked-by: Viresh Kumar <viresh.kumar@xxxxxxxxxx> > > > > > Signed-off-by: Sudeep Holla <sudeep.holla@xxxxxxx> > > > > > > > > Hi Sudeep/Viresh/Rob, > > > > > > > > I noticed this binding recently got accepted, so I guess I have missed > > > > the opportunity to provide you with a few comments. > > > > > > > > > > Sorry for not cc-ing you, wasn't aware of the below mentioned intersection, > > > so assumed you are not one of the interested parties. > > > > > > > In any case, I would like to ask a few questions. In particular, am I > > > > trying to understand why the power-domains bindings [1] can't be used > > > > for this? > > > > > > > > > > One reason I can think of is on some platforms, the power domains are > > > completely controlled by the firmware and not exposed to the OSPM. > > > This is mostly applicable for CPU devices(Platform co-ordinated PSCI) > > > > See below. > > > > > > > > > The power-domains are capable of dealing with "performance" through > > > > the "operating-points-v2" DT property, which maps to the generic OPP > > > > bindings [2]. I wonder why that isn't sufficient here? Can you please > > > > elaborate? > > > > > > > > > > Even if the power domains are exposed to the OSPM, the OPPs can be > > > firmware enumerated rather than DT. Not sure if it is possible to > > > represent such systems in the above mentioned bindings. IIUC, the genpd > > > uses clock and regulator apis to drive the performance, but these > > > platforms have f/w interface to drive the OPPs(abstracted). > > > > Genpd doesn't operate on clock rates or voltage levels. Instead > > "performance" is just an integer value for genpd. What a performance > > index means, is genpd provider specific. > > > > Understood. > > > In other words, it becomes the responsibility for the genpd provider > > to map a performance state index to an OPP, for example. So far, > > providers have used the generic OPP DT bindings to do this, but for > > sure, we don't have to limit ourselves to this. So, if OPP tables can > > be enumerated by FW, rather than specified in DT, that should > > certainly be possible to support. > > > > BTW, these are genpd provider callbacks, that needs to be implemented > > to let it control performance. Perhaps that helps to understand > > things. > > > > int (*set_performance_state)(struct generic_pm_domain *genpd, unsigned > > int state); > > unsigned int (*opp_to_performance_state)(struct generic_pm_domain > > *genpd, struct dev_pm_opp *opp); > > > > Looks like this can be used for devices but what about CPUs ? Yes, that should work. dev_pm_genpd_set_performance_state() takes a struct device* as an in-parameter. The struct device to use would typically be the one that you receive from dev_pm_domain_attach_by_name(). We already do this for some other cpufreq drivers, so this works fine. > > > > > > > I am happy to know if there are ways to support such systems with the > > > options you have mentioned above. > > > > As far as I understand, the "performance domains" DT bindings that > > $subject patch introduces, allows us to group devices into domains, to > > let them be "performance controlled" together. Right? > > > > Or independently. It doesn't matter. > > > Unless I am missing something, it looks like power domains DT bindings > > already offer this for us. Yes, certainly, the DT doc [1] needs an > > updated description to better explain this, but other than that we > > should be fine, don't you think? > > > > As I mentioned about, the main question is what if firmware doesn't > want to expose power domain details to OSPM like PC co-ordinated PSCI > idle states while it wants to either group CPUs or leave them as > individual in order to get per-CPU DVFS requests and aggregate them > in the firmware. It does something similar for idle states already. Yes, that can be modeled too. Just let each CPU node point to its own separate power-domain and also *don't* model the parent power-domain, instead leave this to be managed by the FW. Kind regards Uffe
