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?"
