On 06/15/2015 10:20 PM, Ingo Molnar wrote: >> Actually, ecx and r11 need to be loaded first. They are not so much >> "restored" >> as "prepared for SYSRET insn". Every cycle lost in loading these delays >> SYSRET. >> [...] > > So in the typical case they will still be cached, and so their max latency > should > be around 3 cycles.
If syscall flushes caches (say, a large read), or sleeps and CPU schedules away, then pt_regs->ip,flags are evicted and need to be reloaded. > In fact because they are memory loads, they don't really have dependencies, > they should be available to SYSRET almost immediately, They depend on the memory data. > i.e. within a cycle - and > there's no reason to believe why these loads wouldn't pipeline properly and > parallelize with the many other things SYSRET has to do to organize a return > to > user-space, before it can actually use the target RIP and RFLAGS. This does not sound right. If it takes, say, 20 cycles to pull data from e.g. L3 cache to ECX, then SYSRET can't possibly complete sooner than in 20 cycles. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
