On 10/15/2013 02:27 PM, Jakub Jelinek wrote: > On Tue, Oct 15, 2013 at 02:16:28PM -0400, Carlos O'Donell wrote: >> There is no effective security difference between accessing the randomized >> stack guard value from a global variable or a value stored in the dynamic >> thread vector. >> >> It is only a performance optimization. The choice of a global variable vs. >> DTV offset has only to do with the speed of access of the stack guard. > > DTV access is of course going to be expensive, after all, that is a function > call, the question was about reserving a word at fixed constant offset from > the TLS base and how expensive that is vs. global variable access. > For soft FP I guess global variable access must win, for -mtp=cp15 > ]it depends on how fast the mrc instruction is. I talk about DTV, but I should really just say constant offset from TP since that's what I mean. I don't actually mean using a TLS variable. Will Newton tested the global variable access code on soft and hard TP configurations and to quote some initial discussions: https://sourceware.org/ml/libc-ports/2013-09/msg00134.html ~~~ >From a back of the envelope calculation the cost of accessing TLS is one cycle faster than accessing a global in best case (e.g. Cortex-A15), considerably slower in common case (50-60 cycles, Cortex-A9) and slower still in worst case (function call to libgcc and the kernel, ARMv4/ARMv5). ~~~ Therefore for 32-bit ARM it was decided that a global variable was the best choice. For AArch64 it will definitely be a reserved word at a constant offset from the TP since that's going to be fastest. Does that clarify things? Cheers, Carlos. -- devel mailing list devel@xxxxxxxxxxxxxxxxxxxxxxx https://admin.fedoraproject.org/mailman/listinfo/devel Fedora Code of Conduct: http://fedoraproject.org/code-of-conduct
