[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [PATCH for 4.19?] x86/Intel: unlock CPUID earlier for the BSP
On 13.06.2024 18:17, Andrew Cooper wrote: > On 13/06/2024 9:19 am, Jan Beulich wrote: >> Intel CPUs have a MSR bit to limit CPUID enumeration to leaf two. If >> this bit is set by the BIOS then CPUID evaluation does not work when >> data from any leaf greater than two is needed; early_cpu_init() in >> particular wants to collect leaf 7 data. >> >> Cure this by unlocking CPUID right before evaluating anything which >> depends on the maximum CPUID leaf being greater than two. >> >> Inspired by (and description cloned from) Linux commit 0c2f6d04619e >> ("x86/topology/intel: Unlock CPUID before evaluating anything"). >> >> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx> >> --- >> While I couldn't spot anything, it kind of feels as if I'm overlooking >> further places where we might be inspecting in particular leaf 7 yet >> earlier. >> >> No Fixes: tag(s), as imo it would be too many that would want >> enumerating. > > I also saw that go by, but concluded that Xen doesn't need it, hence why > I left it alone. > > The truth is that only the BSP needs it. APs sort it out in the > trampoline via trampoline_misc_enable_off, because they need to clear > XD_DISABLE in prior to enabling paging, so we should be taking it out of > early_init_intel(). Except for the (odd) case also mentioned to Roger, where the BSP might have the bit clear but some (or all) AP(s) have it set. > But, we don't have an early BSP-only early hook, and I'm not overwhelmed > at the idea of exporting it from intel.c > > I was intending to leave it alone until I can burn this whole > infrastructure to the ground and make it work nicely with policies, but > that's not a job for this point in the release... This last part reads like the rest of your reply isn't an objection to me putting this in with Roger's R-b, but it would be nice if you could confirm this understanding of mine. Without this last part it, especially the 2nd from last paragraph, certainly reads a little like an objection. Jan
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