On 3/30/21 10:56 AM, Len Brown wrote: > On Tue, Mar 30, 2021 at 1:06 PM Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote: >>> On Mar 30, 2021, at 10:01 AM, Len Brown <lenb@xxxxxxxxxx> wrote: >>> Is it required (by the "ABI") that a user program has everything >>> on the stack for user-space XSAVE/XRESTOR to get back >>> to the state of the program just before receiving the signal? >> The current Linux signal frame format has XSTATE in uncompacted format, >> so everything has to be there. >> Maybe we could have an opt in new signal frame format, but the details would need to be worked out. >> >> It is certainly the case that a signal should be able to be delivered, run “async-signal-safe” code, >> and return, without corrupting register contents. > And so an an acknowledgement: > > We can't change the legacy signal stack format without breaking > existing programs. The legacy is uncompressed XSTATE. It is a > complete set of architectural state -- everything necessary to > XRESTOR. Further, the sigreturn flow allows the signal handler to > *change* any of that state, so that it becomes active upon return from > signal. One nit with this: XRSTOR itself can work with the compacted format or uncompacted format. Unlike the XSAVE/XSAVEC side where compaction is explicit from the instruction itself, XRSTOR changes its behavior by reading XCOMP_BV. There's no XRSTORC. The issue with using the compacted format is when legacy software in the signal handler needs to go access the state. *That* is what can't handle a change in the XSAVE buffer format (either optimized/XSAVEOPT, or compacted/XSAVEC).
