Messages in this thread | | | Date | Tue, 16 Jun 2009 10:33:43 +0200 | From | Ingo Molnar <> | Subject | Re: [tip:perfcounters/core] perf_counter: x86: Fix call-chain support to use NMI-safe methods |
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* Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca> wrote:
> I am not asking for the pf handler to handle every possible kind > of fault recursively. Just to keep the in-kernel page fault > related code for vmalloc (and possibly for prefetch ?) paths > NMI-reentrant : > > void do_page_fault(struct pt_regs *regs, unsigned long error_code) > > address = read_cr2();
Why would this be needed? We read the cr2 as the first thing in do_page_fault(). It can be destroyed and re-faulted at will after that point, it wont matter a bit - we have already read it.
The only window to be careful about wrt. cr2 is the small window starting at <page_fault>, leading into <do_page_fault>:
ffffffff8154085f <do_page_fault>: ffffffff8154085f: 55 push %rbp ffffffff81540860: 48 89 e5 mov %rsp,%rbp ffffffff81540863: 41 57 push %r15 ffffffff81540865: 41 56 push %r14 ffffffff81540867: 49 89 f6 mov %rsi,%r14 ffffffff8154086a: 41 55 push %r13 ffffffff8154086c: 49 89 fd mov %rdi,%r13 ffffffff8154086f: 41 54 push %r12 ffffffff81540871: 53 push %rbx ffffffff81540872: 48 83 ec 18 sub $0x18,%rsp ffffffff81540876: 65 4c 8b 3c 25 00 b0 mov %gs:0xb000,%r15 ffffffff8154087d: 00 00 ffffffff8154087f: 49 8b 87 48 02 00 00 mov 0x248(%r15),%rax ffffffff81540886: 48 89 45 d0 mov %rax,-0x30(%rbp) ffffffff8154088a: 48 83 c0 60 add $0x60,%rax ffffffff8154088e: 48 89 45 c8 mov %rax,-0x38(%rbp) ffffffff81540892: 0f 18 08 prefetcht0 (%rax) ffffffff81540895: 41 0f 20 d4 mov %cr2,%r12
Look how early we read out cr2 - after trapping we read it after about 40 straight instructions, with no other function call inbetween. Only an NMI (or an MCE and similar deep-atomic contexts) can get in that window.
( Btw., a sidenote: the prefetcht0 right before the cr2 read is a real bug. Prefetches can sometimes generate false faults and thus destroy the value cr2. I'll send a patch for that soon. )
Ingo
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