* Mathieu Desnoyers ([EMAIL PROTECTED]) wrote:
> * Alexey Dobriyan ([EMAIL PROTECTED]) wrote:
> > On Fri, Jul 13, 2007 at 09:26:43PM -0400, Mathieu Desnoyers wrote:
> > > Use the faster immediate values for F00F bug handling in do_page_fault.
> >
> > > --- linux-2.6-lttng.orig/arch/i386/mm/fault.c
> > > +++ linux-2.6-lttng/arch/i386/mm/fault.c
> > > @@ -492,7 +493,7 @@
> > > /*
> > > * Pentium F0 0F C7 C8 bug workaround.
> > > */
> > > - if (boot_cpu_data.f00f_bug) {
> > > + immediate_if (&f00f_bug_fix) {
> >
> > This code is not called during normal pagefaults and even during invalid
> > userspace accesses.
> >
> > Out of curiosity, I inserted printk() at this place to see where I was
> > wrong. I got only two hits:
> >
> > Checking if this processor honours the WP bit even in supervisor
> > mode... do_page_fault:
> > Freeing unused kernel memory: 116k freed
> > do_page_fault:
> >
> > Resume: nobody gives a fuck about performance of this particular if,
> > so conversion it totally pointless.
> >
>
> Interesting investigation, let's push it further:
>
> instrumenting the f00f test site with a printk, I get:
>
> [ 0.000000] Checking if this processor honours the WP bit even in
> supervisor mode... TEST: would test f00f bug at vadd ffecc000, eip c011928e
> [ 0.000000] Ok.
> ... and (whenever xdm restarts) :
> [ 64.768165] TEST: would test f00f bug at vadd 00000000, eip c0237596
> [ 64.787136] TEST: would test f00f bug at vadd 0000004c, eip c02375a2
>
> Those EIPs are:
>
> 0xc011928e <do_test_wp_bit+20>: mov %cl,0xffecd000(%edx)
> -> Will trigger fixup_exception.
>
> 0xc0237596 <__copy_from_user_ll+53>: rep movsl %ds:(%esi),%es:(%edi)
> 0xc02375a2 <__copy_from_user_ll+65>: mov 0x20(%esi),%eax
> -> Those look like user-space programs that gave NULL pointers to kernel
> system calls.
>
> I agree with you that this is not a "hot path". It was mostly a
> straightforward test conversion.
>
> Mathieu
Actually, what is even more weird is that my system is configured to
output the segfaults, i.e. a small user-space program that does a memory
read of *(char*)NULL causes:
[ 1413.164212] null[3740]: segfault at 00000000 eip 0804836a esp
bffde2c0 error 4
But there is no message for the __copy_from_user_ll fault, so I wonder
1 - has the process been killed ?
2 - Is it just the printk that is missing ?
Mathieu
--
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
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