Re: [tip:perfcounters/core] perf_counter: x86: Fix call-chainsupport to use NMI-safe methods

[Ingo Molnar]

* Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxx> 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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