On March 20, 2026 8:50:54 AM PDT, Dave Hansen <[email protected]> wrote: >On 3/20/26 08:47, Andrew Cooper wrote: >>> First, CPUID doesn't tell you if FRED is in use. Is it even on by >>> default yet? There might not be a better way to do this than checking >>> CPUID, but checking CPUID is imprecise at best. >> A reliable way to distinguish IDT and FRED mode is to: >> >> 1) Load $3 into %fs (x86_64) or %gs (i386) (i.e. whichever isn't thread >> local stoage) >> 2) execute a breakpoint, ignore the signal >> 3) Look to see whether %fs/%gs holds 3 or 0 >> >> IRET has a fun behaviour where it zeroes NULL selectors even if they had >> a non-zero RPL. >> >> ERETU doesn't do this; Andy Luto and I asked for this minor information >> leak to be removed, and Intel agreed as it served no purpose anyone >> could identify. >> >> As a consequence, you can use it to determine whether the kernel used >> IRET or ERET to return back to userspace. > >I was thinking of just grepping /proc/cpuinfo for "fred", but that >sounds much more fun! :)
This is a great example of why we need the patchset to clear compiled-out flags.
