KVM: x86: Preserve DR6.BLD (Bus Lock Detect) when delivering #DB payload

Preserve Bus Lock Detect if it isn't present in the payload.  Unlike RTM,
which is explicitly modified on every #DB,  DR6.BLD is modified if and only
if the #DB is due to a (coincident) Bus Lock Detect.

Per Intel's SDM:

  Other debug exceptions do not modify this bit.

And AMD's APM

  All other #DB exceptions leave DR6[BLD] unmodified,

Restore the explicit RTM handling that was clobbered by commit
9a3ecd5e2aa1 ("KVM: X86: Rename DR6_INIT to DR6_ACTIVE_LOW"), as the
semantics of any given bit are feature specific, i.e. are obviously not
consistent for active-high versus active-low.  In other words, while the
bug was technically introduced when support for Bus Lock Detect was added,
it's really that attempt to genericize handling of active-low bits that set
KVM up to fail.

Fixes: e8ea85fb280e ("KVM: X86: Add support for the emulation of DR6_BUS_LOCK bit")
Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20260930193634.3415089-2-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

0 files changed, 0 insertions, 0 deletions