linux-kernel - Re: [tip:perfcounters/core] perf_counter: x86: Fix call-chain support to use NMI-safe methods

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Date:	Tue, 16 Jun 2009 10:19:56 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	mingo@...hat.com, paulus@...ba.org, acme@...hat.com,
	linux-kernel@...r.kernel.org, penberg@...helsinki.fi,
	vegard.nossum@...il.com, efault@....de, jeremy@...p.org,
	npiggin@...e.de, tglx@...utronix.de,
	linux-tip-commits@...r.kernel.org
Subject: Re: [tip:perfcounters/core] perf_counter: x86: Fix call-chain
	support to use NMI-safe methods

* Ingo Molnar (mingo@...e.hu) wrote:
> 
> * Mathieu Desnoyers <mathieu.desnoyers@...ymtl.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.
> 

With respect to cr2, yes, this is the only window we care about.
However, the rest of vmalloc_fault() must be audited for other non
nmi-suitable data structure use (e.g. "current"), which I did in the
past.

My intent was just to respond to Peter's concerns by showing that the
part of page fault handler which needs to be NMI-reentrant is really not
that big.

Mathieu

> 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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-- 
Mathieu Desnoyers
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