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    SubjectRe: [v6 PATCH 21/21] selftests/x86: Add tests for User-Mode Instruction Prevention
    On Tue, Mar 7, 2017 at 4:32 PM, Ricardo Neri
    <ricardo.neri-calderon@linux.intel.com> wrote:
    > Certain user space programs that run on virtual-8086 mode may utilize
    > instructions protected by the User-Mode Instruction Prevention (UMIP)
    > security feature present in new Intel processors: SGDT, SIDT and SMSW. In
    > such a case, a general protection fault is issued if UMIP is enabled. When
    > such a fault happens, the kernel catches it and emulates the results of
    > these instructions with dummy values. The purpose of this new
    > test is to verify whether the impacted instructions can be executed without
    > causing such #GP. If no #GP exceptions occur, we expect to exit virtual-
    > 8086 mode from INT 0x80.
    >
    > The instructions protected by UMIP are executed in representative use
    > cases:
    > a) the memory address of the result is given in the form of a displacement
    > from the base of the data segment
    > b) the memory address of the result is given in a general purpose register
    > c) the result is stored directly in a general purpose register.
    >
    > Unfortunately, it is not possible to check the results against a set of
    > expected values because no emulation will occur in systems that do not have
    > the UMIP feature. Instead, results are printed for verification.

    You could pre-initialize the result buffer to a bunch of non-matching
    values (1, 2, 3, ...) and then check that all the invocations of the
    same instruction gave the same value.

    If you do this, maybe make it a follow-up patch -- see other email.

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