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

Avi Kivity
2010-07-18T11:05:09Z

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