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SubjectRe: [patch 1/2] x86_64 page fault NMI-safe
Hi Linus,

I modified your code, intenting to handle the fake NMI entry gracefully given
that NMIs are not necessarily disabled at the entry point. It uses a "need fake
NMI" flag rather than playing games with CS and faults. When a fake NMI is
needed, it simply jumps back to the beginning of regular nmi code. NMI exit code
and fake NMI entry are made reentrant with respect to NMI handler interruption
by testing, at the very beginning of the NMI handler, if a NMI is nested over
the whole nmi_atomic .. nmi_atomic_end code region. This code assumes NMIs have
a separate stack.

This code is still utterly untested and might eat your Doritos, only provided
for general enjoyment.



# Two per-cpu variables: a "are we nested" flag (one byte).
# a "do we need to execute a fake NMI" flag (one byte).
# The %rsp at which the stack copy is saved is at a fixed address, which leaves
# enough room at the bottom of NMI stack for the "real" NMI entry stack. This
# assumes we have a separate NMI stack.
# The NMI stack copy top of stack is at nmi_stack_copy.
# The NMI stack copy "rip" is at nmi_stack_copy_rip, which is set to
# nmi_stack_copy-32.
# Test if nested over atomic code.
cmpq $nmi_atomic,0(%rsp)
jae nmi_addr_is_ae
# Test if nested over general NMI code.
cmpb $0,%__percpu_seg:nmi_stack_nesting
jne nmi_nested_set_fake_and_return
# create new stack
# 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).

# Save %rax on top of the stack (need to temporarily use it)
pushq %rax
movq %rsp, %rax
movq $nmi_stack_copy,%rsp

# copy the five words of stack info. rip starts at 8+0(%rax).
pushq 8+32(%rax) # ss
pushq 8+24(%rax) # rsp
pushq 8+16(%rax) # eflags
pushq 8+8(%rax) # cs
pushq 8+0(%rax) # rip
movq 0(%rax),%rax # restore %rax

# and set the nesting flags
movb $0xff,%__percpu_seg:nmi_stack_nesting

# regular NMI code goes here, and can take faults,
# because this sequence now has proper nested-nmi
# handling

# An NMI nesting over the whole nmi_atomic .. nmi_atomic_end region will
# be handled specially. This includes the fake NMI entry point.
cmpb $0,%__percpu_seg:need_fake_nmi
jne fake_nmi
movb $0,%__percpu_seg:nmi_stack_nesting

# This is the fake NMI entry point.
movb $0x0,%__percpu_seg:need_fake_nmi
jmp regular_nmi_code

# Make sure the address is in the nmi_atomic range and in CS segment.
cmpq $nmi_atomic_end,0(%rsp)
jae is_unnested_nmi
# The saved rip points to the final NMI iret. Check the CS segment to
# make sure.
cmpw $__KERNEL_CS,8(%rsp)
jne is_unnested_nmi

# This is the case when we hit just as we're supposed to do the atomic code
# of a previous nmi. We run the NMI using the old return address that is still
# on the stack, rather than copy the new one that is bogus and points to where
# the nested NMI interrupted the original NMI handler!
# Easy: just set the stack pointer to point to the stack copy, clear
# need_fake_nmi (because we are directly going to execute the requested NMI) and
# jump to "nesting flag set" (which is followed by regular nmi code execution).
movq $nmi_stack_copy_rip,%rsp
movb $0x0,%__percpu_seg:need_fake_nmi
jmp set_nmi_nesting

# This is the actual nested case. Make sure we branch to the fake NMI handler
# after this handler is done.
movb $0xff,%__percpu_seg:need_fake_nmi
jmp *(%rsp)

Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.

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