On Tue, Nov 08, 2011 at 09:33:52PM -0800, Kevin Cernekee wrote: > I read through mmu_context.h and the threads from November/December > 2010 a couple of times, and I'm starting to think Maksim's original > approach (don't reset asid_cache(cpu) when warm-restarting a CPU) > makes the most sense. > > The basic issue is that we want to assign unique, strictly increasing > values to each mm's cpu_context(cpu, mm). The per-cpu counter starts > at ASID_FIRST_VERSION (0x100 on R4K), and counts up. Assigning a new > mm the same ASID value as an existing mm on the same CPU is illegal. > Two obvious ways to meet this requirement when hotplugging CPUs are: > > Option #1: Retain the asid_cache(cpu) value across warm restarts. > This is simple and inexpensive. We pick up where we left off, and > whatever existing cpu_context(cpu, mm) values are out there do not > cause any trouble. > > I believe Maksim's original logic (assign ASID_FIRST_VERSION, a > nonzero number, if asid_cache(cpu) == 0) would work correctly as > written, because cpu_data is an array in .bss . It will be 0 until > the CPU is booted, and get_new_mmu_context() ensures that it will > never be 0 again after that. > > Kevin K brought up the idea of a warm restart bitmask so the code > could tell whether asid_cache(cpu) was valid. I'm not sure that this > would be required. Neither do I. > I think we can also get away with not explicitly preserving EntryHi, > since switch_mm() and activate_mm() will set it anyway. > > Option #2: When warm restarting a CPU, set asid_cache(cpu) to > ASID_FIRST_VERSION again. And at some point (cpu_up or cpu_down), > iterate through all processes to set cpu_context(cpu, mm) to something > that will not conflict with a newly assigned ASID. This is what the > most recent patch did. It gets the job done, but it's more work than > what is really needed. > > Please let me know your thoughts... I still like the original patch https://patchwork.linux-mips.org/patch/1797/ I'd apply it right away but I'm going to hold off for a day just to give Kevin and maybe others a chance to comment. Ralf
