On Tue, Dec 12, 2017 at 9:43 AM, Peter Zijlstra <pet...@infradead.org> wrote: > On Tue, Dec 12, 2017 at 09:21:10AM -0500, Ilia Mirkin wrote: >> The "thing" being mmiotrace, or the "thing" being page-unaligned addresses? > > mmiotrace > >> If the former, its primary use-case is for snooping on the NVIDIA >> proprietary GPU driver in order to figure out how to drive the >> underlying hardware. The driver does ioremap's to get at PCI space, >> which mmiotrace "steals" and provides pages without a present bit set, >> along with a fault handler. When the fault handler is hit, it >> reinstates the faulting page, and single-steps the faulting >> instruction > > At which point you have valid page-tables and another CPU can access > that page too. > >> reading the before/after regs to determine what happened >> (doesn't work universally, but enough for instructions used for PCI >> MMIO accesses). See mmio-mod.c::pre and post (the latter is called >> from the debug handler). > > And after that you only invalidate the TLBs for the CPU that took the > initial fault, leaving possibly stale TLBs on other CPUs. > > > So this 'thing' has huge gaping SMP holes in.
Sure does! Probably why the following happens when mmiotrace is enabled: void enable_mmiotrace(void) { mutex_lock(&mmiotrace_mutex); if (is_enabled()) goto out; if (nommiotrace) pr_info("MMIO tracing disabled.\n"); kmmio_init(); enter_uniprocessor(); spin_lock_irq(&trace_lock); atomic_inc(&mmiotrace_enabled); spin_unlock_irq(&trace_lock); pr_info("enabled.\n"); out: mutex_unlock(&mmiotrace_mutex); }