On 5/20/21 2:41 PM, Len Brown wrote: > So the questions are: > 1. who calls it -- a call/thread or process? the application? a > library -- which library? > 2. is it optional, or mandatory? > 3. if it is mandatory, what is the best way to enforce it? > 4. should we have a "release" system call too? > > 1. Every thread needs a context switch buffer. Does every thread make > the system call? It seems sort of awkward for a library to always > make a system call before doing a TMUL. It would be functionally > harmless, but it would add latency to an otherwise low-latency > operation. If some central library does it, and caches that it has > done it before, then it would be ugly, but at least it would remove an > unnecessary user/kernel transition. Our system calls are *REALLY* fast. We can even do a vsyscall for this if we want to get the overhead down near zero. Userspace can also cache the "I did the prctl()" state in thread-local storage if it wants to avoid the syscall. > 2. If it is optional, then v5 is code complete -- because it allows > you to allocate either explicitly via prtcl, or transparently via #NM. It needs to be mandatory. If it's not, then nobody will use it, and they'll suffer the dreaded SIGSEGV-on-vmalloc()-failure and start filing bug reports. > 3. If it is mandatory, then we should re-purpose the XFD mechanism: > app starts with XFD armed, by default > if app touches AMX before prctl, it takes a signal (and dies). > When app calls prctl, allocate buffer disarm XFD for that app (exactly > what #NM trap does today). Yes, that sounds like a good use of XFD. > 4. I don't see a justification for a release concept, but it is > possible -- though sort of sticky with possible nested calls from > combinations of apps and libraries. If that were sorted out by a > central library, then the actual system call on the last release per > thread would re-arm XFD to prevent access until the next explicit > request. Unclear if it is important that the kernel actually do the > free -- some things might run faster if we keep it around... I think would be more of a get/put model rather than an allocate/free model. The "put" could effectively be a noop for now. But, if we don't put this in the ABI up front, we can't add it later. That means that we could never add a lazy-free, even if we wanted to.
