Re: [patch 1/2] x86_64 page fault NMI-safe

[Avi Kivity <avi@redhat.com>]

On 07/15/2010 04:23 AM, Linus Torvalds wrote:
> On Wed, Jul 14, 2010 at 3:37 PM, Linus Torvalds
> <torvalds@linux-foundation.org>  wrote:
>    
>> I think the %rip check should be pretty simple - exactly because there
>> is only a single point where the race is open between that 'mov' and
>> the 'iret'. So it's simpler than the (similar) thing we do for
>> debug/nmi stack fixup for sysenter that has to check a range.
>>      
> So this is what I think it might look like, with the %rip in place.
> And I changed the "nmi_stack_ptr" thing to have both the pointer and a
> flag - because it turns out that in the single-instruction race case,
> we actually want the old pointer.
>
> Totally untested, of course. But _something_ like this might work:
>
> #
> # Two per-cpu variables: a "are we nested" flag (one byte), and
> # a "if we're nested, what is the %rsp for the nested case".
> #
> # The reason for why we can't just clear the saved-rsp field and
> # use that as the flag is that we actually want to know the saved
> # rsp for the special case of having a nested NMI happen on the
> # final iret of the unnested case.
> #
> nmi:
> 	cmpb $0,%__percpu_seg:nmi_stack_nesting
> 	jne nmi_nested_corrupt_and_return
> 	cmpq $nmi_iret_address,0(%rsp)
> 	je nmi_might_be_nested
> 	# create new stack
> is_unnested_nmi:
> 	# Save some space for nested NMI's. The exception itself
> 	# will never use more space, but it might use less (since
> 	# if will be a kernel-kernel transition). But the nested
> 	# exception will want two save registers and a place to
> 	# save the original CS that it will corrupt
> 	subq $64,%rsp
>
> 	# copy the five words of stack info. 96 = 64 + stack
> 	# offset of ss.
> 	pushq 96(%rsp)   # ss
> 	pushq 96(%rsp)   # rsp
> 	pushq 96(%rsp)   # eflags
> 	pushq 96(%rsp)   # cs
> 	pushq 96(%rsp)   # rip
>
> 	# and set the nesting flags
> 	movq %rsp,%__percpu_seg:nmi_stack_ptr
> 	movb $0xff,%__percpu_seg:nmi_stack_nesting
>
>    

By trading off some memory, we don't need this trickery.  We can 
allocate two nmi stacks, so the code becomes:

nmi:
     cmpb $0, %__percpu_seg:nmi_stack_nesting
     je unnested_nmi
     cmpq $nmi_iret,(%rsp)
     jne unnested_nmi
     cmpw $__KERNEL_CS,8(%rsp)
     jne unnested_nmi
     popf
     retfq
unnested_nmi:
     xorq $(nmi_stack_1 ^ nmi_stack_2),%__percpu_seg:tss_nmi_ist_entry
     movb $1, __percpu_seg:nmi_stack_nesting
regular_nmi:
     ...
regular_nmi_end:
     movb $0, __percpu_seg:nmi_stack_nesting
nmi_iret:
     iretq




-- 
error compiling committee.c: too many arguments to function