[Bug 219787] Guest's applications crash with EXCEPTION_SINGLE_STEP (0x80000004)

[bugzilla-daemon@kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=219787

--- Comment #16 from Ravi Bangoria (ravi.bangoria@amd.com) ---
(In reply to Sean Christopherson from comment #13)

> And does AMD do _any_ testing?  This doesn't even require a full reproducer,
> e.g. the existing debug KVM-Unit-Test fails on my system (Turin) without ever
> generating a split/bus lock.  AFAICT, the CPU is reporting bus locks in DR6
> on
> #DBs that are most definitely not due to bus locks.

It seems, the CPU is preserving SW written DR6[BusLockDetected] while
generating the #DB when the CPL is 0 and DEBUGCTL[BusLockTrapEn] is set.

Since most of the x86/debug.c KUT tests clears DR6[BusLockDetected] before
executing the test, the bit remains cleared at the exception time which causes
tests to fail.

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