Hi Doug, On Mon, Dec 09, 2024 at 09:43:10AM -0800, Douglas Anderson wrote: > > Since Qualcomm CPUs are all derivatives of ARM cores they all have > unique MIDR values. This means that the tables listing necessary > Spectre mitigations need special entries for them. However, those > entries are not present and that means that some Spectre mitigations > are lacking for Qualcomm CPUs. > > I've made an attempt at **GUESSING** what the right patches should be > to enable mitigations for Qualcomm CPUs. This is mostly me searching > the web to figure out what ARM cores various Qualcomm cores are based > off of. > > These patches get more and more sketchy as the series progresses and I > have noted that the later patces DON'T EVEN COMPILE. I have included > them to make it obvious that I think these cores are affected even if > I don't have all the right information to mitigate them. Hopefully > Qualcomm can come and fix this mess for me. > > I'll note that I am certainly no expert on Spectre. Mostly I ended up > here running `lscpu` on a device and noticing that it thought that it > wasn't affected by Spectre v2 when I thought it was. Whilst only Qualcomm can say definitively whether or not they are affected (and what values of 'k' are required for the loop-based workarounds), I can't help but wonder whether the current mitigation code is structured the wrong way around in this case. It looks to me like we don't have a way to identify a "vulnerable" CPU for Spectre-BHB; either a CPU has some sort of mitigation or it's unaffected. That means that there's very little incentive for vendors to add their CPUs to one of the lists -- if they do nothing, userspace is told that everything is golden and they don't pay the performance hit of a workaround! So I think we should consider turning this on its head and assume that CPUs we don't know about are vulnerable, having a list of unaffected cores that predate the introduction of CSV2.3 which can be queried by is_spectre_bhb_affected(). We can do that without the assistance of the CPU vendors. Does that make sense, or did I miss something? Will
