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

    Linus
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