linux-kernel - [PATCH v2 0/2] x86/traps: Fix DR6/DR7 initialization

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Message-ID: <20250617073234.1020644-1-xin@zytor.com>
Date: Tue, 17 Jun 2025 00:32:32 -0700
From: "Xin Li (Intel)" <xin@...or.com>
To: linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc: tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com,
        seanjc@...gle.com, pbonzini@...hat.com, peterz@...radead.org,
        sohil.mehta@...el.com, brgerst@...il.com, tony.luck@...el.com,
        fenghuay@...dia.com
Subject: [PATCH v2 0/2] x86/traps: Fix DR6/DR7 initialization

Sohil reported seeing a split lock warning when running a test that
generates userspace #DB:

  x86/split lock detection: #DB: sigtrap_loop_64/4614 took a bus_lock trap at address: 0x4011ae

We investigated the issue and figured out:

  1) The warning is a false positive.

  2) It is not caused by the test itself.

  3) It occurs even when Bus Lock Detection (BLD) is disabled.

  4) It only happens on the first #DB on a CPU.

And the root cause is, at boot time, Linux zeros DR6.  This leads to
different DR6 values depending on whether the CPU supports BLD:

  1) On CPUs with BLD support, DR6 becomes 0xFFFF07F0 (bit 11, DR6.BLD,
     is cleared).

  2) On CPUs without BLD, DR6 becomes 0xFFFF0FF0.

Since only BLD-induced #DB exceptions clear DR6.BLD and other debug
exceptions leave it unchanged, even if the first #DB is unrelated to
BLD, DR6.BLD is still cleared.  As a result, such a first #DB is
misinterpreted as a BLD #DB, and a false warning is triggerred.

Fix the bug by initializing DR6 by writing its architectural reset
value at boot time.

DR7 suffers from a similar issue.  We apply the same fix.

This patch set is based on tip/x86/urgent branch as of today.

Changes in v2:
*) Use debug register indexes rather than DR_* macros (PeterZ and Sean).
*) Use DR7_FIXED_1 as the architectural reset value of DR7 (Sean).
*) Move the DR6 fix patch to the first of the patch set to ease backporting.

Xin Li (Intel) (2):
  x86/traps: Initialize DR6 by writing its architectural reset value
  x86/traps: Initialize DR7 by writing its architectural reset value

 arch/x86/include/asm/debugreg.h      | 14 ++++++++----
 arch/x86/include/asm/kvm_host.h      |  2 +-
 arch/x86/include/uapi/asm/debugreg.h |  7 +++++-
 arch/x86/kernel/cpu/common.c         | 17 ++++++--------
 arch/x86/kernel/kgdb.c               |  2 +-
 arch/x86/kernel/process_32.c         |  2 +-
 arch/x86/kernel/process_64.c         |  2 +-
 arch/x86/kernel/traps.c              | 34 +++++++++++++++++-----------
 arch/x86/kvm/x86.c                   |  4 ++--
 9 files changed, 50 insertions(+), 34 deletions(-)

base-commit: 594902c986e269660302f09df9ec4bf1cf017b77
-- 
2.49.0