The cpuid bit gets twiddled... On September 18, 2014 5:23:40 PM PDT, Andy Lutomirski <l...@amacapital.net> wrote: >On Sep 18, 2014 5:13 PM, "Henrique de Moraes Holschuh" <h...@hmh.eng.br> >wrote: >> >> On Thu, 18 Sep 2014, Henrique de Moraes Holschuh wrote: >> > On Thu, 18 Sep 2014, H. Peter Anvin wrote: >> > > We should, but this is also part of why we want the early ucode >capability. >> > >> > Well, yes. But that won't help the several stable and LTS distros >with >> > kernels without early ucode update support. >> >> Here's a plan that might work, pending actually checking the >libpthread TSX >> code to make sure it keys on /proc/cpuinfo flags: > >Surely it checks cpuid directly, though. > >Can we twiddle the cpuid bit? I never noticed any way in the docs to >do it, but if BIOS has such an ability, maybe we do, too. I wonder if >there's anything semi-documented in biosbits, or if we could just >reverse-engineer it. > >--Andy > >> >> Add a cpu quirk, triggered by the Haswell cpuids, to force-disable >hle on >> the affected processors. >> >> This will work around the x86_capability capability issue (which >should >> still be fixed, anyway), and it should also get userspace to stay >away from >> TSX, therefore also working around the worst issue (processes getting >> SIGILL). >> >> This will disable the "user may ask the BIOS to keep TSX enabled" >> anti-feature, though. This drawback can be avoided, but only if a >future >> microcode update won't re-disable hle when the BIOS enabled it. For >now, I >> suggest that we decree that "hle is toast" for the current Haswells >and add >> back ways to enable it for testing when we know more about it. >> >> -- >> "One disk to rule them all, One disk to find them. One disk to >bring >> them all and in the darkness grind them. In the Land of Redmond >> where the shadows lie." -- The Silicon Valley Tarot >> Henrique Holschuh
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