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
