Messages in this thread |  | | | Date | Sun, 18 Jul 2010 10:36:42 -0700 | | Subject | Re: [patch 1/2] x86_64 page fault NMI-safe | | From | Linus Torvalds <> |
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On Sun, Jul 18, 2010 at 4:03 AM, Avi Kivity <avi@redhat.com> wrote: > > By trading off some memory, we don't need this trickery. We can allocate > two nmi stacks, so the code becomes:
I really don't think you need even that. See earlier in the discussion about how we could just test %rsp itself. Which makes all the %rip testing totally unnecessary, because we don't even need any flags,and we have no races because %rsp is atomically changed with taking the exception.
Lookie here, the %rsp comparison really isn't that hard:
nmi: pushq %rax pushq %rdx movq %rsp,%rdx # current stack top movq 40(%rsp),%rax # old stack top xor %rax,%rdx # same 8kB aligned area? shrq $13,%rdx # ignore low 13 bits je it_is_a_nested_nmi # looks nested.. non_nested: ... ... ok, we're not nested, do normal NMI handling ... ... popq %rdx popq %rax iret
it_is_a_nested_nmi: cmpw $0,48(%rsp) # double-check that it really was a nested exception jne non_nested # from user space or something.. # this is the nested case # NOTE! NMI's are blocked, we don't take any exceptions etc etc addq $-160,%rax # 128-byte redzone on the old stack + 4 words movq (%rsp),%rdx movq %rdx,(%rax) # old %rdx movq 8(%rsp),%rdx movq %rdx,8(%rax) # old %rax movq 32(%rsp),%rdx movq %rdx,16(%rax) # old %rflags movq 16(%rsp),%rdx movq %rdx,24(%rax) # old %rip movq %rax,%rsp popq %rdx popq %rax popf ret $128 # restore %rip and %rsp
doesn't that look pretty simple?
NOTE! OBVIOUSLY TOTALLY UNTESTED!
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