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J. Lu" , X86 ML , LKML , "Bae, Chang Seok" , Carlos O'Donell , Rich Felker , libc-alpha References: <87a6qqi064.fsf@mid.deneb.enyo.de> <87blb5d7zx.fsf@mid.deneb.enyo.de> From: Andrew Cooper Subject: Re: Why does glibc use AVX-512? 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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 >> >> >> 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 >> > It's worth noting that recent microcode updates have make RTM > considerably less likely to actually work on many parts. It's > possible you should just disable it. :( For a variety of errata and speculative security reasons, hypervisors now have the ability to hide/show the HLE/RTM CPUID bits, independently of letting TSX actually work or not. For migration compatibility reasons, you might quite possibly find yourself in a VM which advertises the HLE/RTM bits but will unconditionally abort any transaction. Honestly, if I were you, I'd just leave it to the user to explicitly opt in if they want transactions. ~Andrew