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The undocumented 'icebp' instruction (aka 'int1') works pretty much like 'int3' in the absense of in-circuit probing equipment (except, obviously, that it raises #DB instead of raising #BP), and is used by some validation test-suites as such. But Andy Lutomirski noticed that his test suite acted differently in kvm than on bare hardware. The reason is that kvm used an inexact test for the icebp instruction: it just assumed that an all-zero VM exit qualification value meant that the VM exit was due to icebp. That is not unlike the guess that do_debug() does for the actual exception handling case, but it's purely a heuristic, not an absolute rule. do_debug() does it because it wants to ascribe _some_ reasons to the #DB that happened, and an empty %dr6 value means that 'icebp' is the most likely casue and we have no better information. But kvm can just do it right, because unlike the do_debug() case, kvm actually sees the real reason for the #DB in the VM-exit interruption information field. So instead of relying on an inexact heuristic, just use the actual VM exit information that says "it was 'icebp'". Right now the 'icebp' instruction isn't technically documented by Intel, but that will hopefully change. The special "privileged software exception" information _is_ actually mentioned in the Intel SDM, even though the cause of it isn't enumerated. Reported-by: Andy Lutomirski <luto@kernel.org> Tested-by: Paolo Bonzini <pbonzini@redhat.com> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. Please read the Documentation/process/changes.rst file, as it contains the requirements for building and running the kernel, and information about the problems which may result by upgrading your kernel.