On 24/03/2026 2:33 pm, H. Peter Anvin wrote: > On March 24, 2026 7:08:14 AM PDT, Andrew Cooper <[email protected]> > wrote: >> On 23/03/2026 8:27 pm, H. Peter Anvin wrote: >>> On 2026-03-23 12:17, Andrew Cooper wrote: >>>> This doesn't really test whether FRED is active. It tests whether the >>>> OS is not providing strict backwards compatibility, and I think will >>>> malfunction when there's a hypervisor above Linux providing strict >>>> backwards compatibility. >>>> >>> But that applies equally to IRET, no? If the hypervisor clobbers the segment >>> selector like IRET would in the interest of compatibility then you have the >>> same issue. >> I suppose. I for one don't care to provide that level of compatibility. >> >> But for SYSCALL, what are Linux's plans for CRIU or RR ? I had to fix >> SYSCALL legacy behaviour in Xen for the following case: >> >> * PV guest issues SYSCALL on FRED system. %rcx/%r11 not clobbered >> * Migrate to a non-FRED system >> * Xen uses a real SYSRET instruction to resume execution >> >> >> Here, the guest continues executing at whichever dead variable is in %rcx. >> >> CRIU/RR won't be exactly the same, but will suffer the same class of >> problem when moving between FRED and non-FRED systems. >> >> ~Andrew > "Doctor, it hurts when I PV?"
I'm asking straight up, what is Linux doing to fix this same issue for CRIU/RR? ~Andrew P.S. It's rhetorical, seeing as it's taken between Linux 6.9 and now (2 whole years) for anyone to even run the x86 selftests on a FRED system.
