* Andy Lutomirski: >> > AVX-512 cleared, and programs need to explicitly request enablement. >> > This would allow programs to opt into not saving/restoring across >> > signals or to save/restore in buffers supplied when the feature is >> > enabled. >> >> Isn't XSAVEOPT already able to handle that? >> > > Yes, but we need a place to put the data, and we need to acknowledge > that, with the current save-everything-on-signal model, the amount of > time and memory used is essentially unbounded. This isn't great.
The size has to have a known upper bound, but the save amount can be dynamic, right? How was the old lazy FPU initialization support for i386 implemented? >> Assuming you can make XSAVEOPT work for you on the kernel side, my >> instincts tell me that we should have markup for RTM, not for AVX-512. >> This way, we could avoid use of the AVX-512 registers and keep using >> VZEROUPPER, without run-time transaction checks, and deal with other >> idiosyncrasies needed for transaction support that users might >> encounter once this feature sees more use. But the VZEROUPPER vs RTM >> issues is currently stuck in some internal process issue on my end (or >> two, come to think of it), which I hope to untangle next month. > > Can you elaborate on the issue? This is the bug: vzeroupper use in AVX2 multiarch string functions cause HTM aborts <https://sourceware.org/bugzilla/show_bug.cgi?id=27457> Unfortunately we have a bug (outside of glibc) that makes me wonder if we can actually roll out RTM transaction checks (or any RTM instruction) on a large scale: x86: Sporadic failures in tst-cpu-features-cpuinfo <https://sourceware.org/bugzilla/show_bug.cgi?id=27398#c3> The dynamic RTM check might trap due to this bug. (We have a bit more information about the nature of the bug, currently missing from Bugzilla.) I'm also worried that the new dynamic RTM check in the string functions has a performance impact. Due to its nature, it will be enabled for every program once running on RTM-capable hardware, not just those that actually use RTM.
