>>> "H. Peter Anvin" <hpa@xxxxxxxxx> 11/04/14 9:11 PM >>> >On 11/04/2014 11:45 AM, tip-bot for Jan Beulich wrote: >> x86-64: Use RIP-relative addressing for most per-CPU accesses >> >> Observing that per-CPU data (in the SMP case) is reachable by >> exploiting 64-bit address wraparound (building on the default kernel >> load address being at 16Mb), the one byte shorter RIP-relative >> addressing form can be used for most per-CPU accesses. The one >> exception are the "stable" reads, where the use of the "P" operand >> modifier prevents the compiler from using RIP-relative addressing, but >> is unavoidable due to the use of the "p" constraint (side note: with >> gcc 4.9.x the intended effect of this isn't being achieved anymore, >> see gcc bug 63637). >> >> With the dependency on the minimum kernel load address, arbitrarily >> low values for CONFIG_PHYSICAL_START are now no longer possible. A >> link time assertion is being added, directing to the need to increase >> that value when it triggers. >> > >This description makes very little sense. What matters here is the >virtual address, which is always >= -2 GB. But code at or very close to -2Gb won't be able to reach addresses far enough from address zero; it indeed matters that the lowest code address is far enough away from -2Gb. Jan -- To unsubscribe from this list: send the line "unsubscribe linux-tip-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
|