linux-kernel - [PATCH v1 0/3] x86/traps: Fix DR6/DR7 inintialization

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Message-ID: <20250613070118.3694407-1-xin@zytor.com>
Date: Fri, 13 Jun 2025 00:01:14 -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,
        brgerst@...il.com, tony.luck@...el.com, fenghuay@...dia.com
Subject: [PATCH v1 0/3] x86/traps: Fix DR6/DR7 inintialization

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 identified how the false bus lock detected
warning is generated under certain test conditions:

  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.

Xin Li (Intel) (3):
  x86/traps: Move DR7_RESET_VALUE to <uapi/asm/debugreg.h>
  x86/traps: Initialize DR7 by writing its architectural reset value
  x86/traps: Initialize DR6 by writing its architectural reset value

 arch/x86/coco/sev/core.c             |  1 +
 arch/x86/coco/sev/vc-handle.c        |  1 +
 arch/x86/include/asm/debugreg.h      | 12 +++++-----
 arch/x86/include/asm/sev-internal.h  |  2 --
 arch/x86/include/uapi/asm/debugreg.h |  9 ++++++-
 arch/x86/kernel/cpu/common.c         | 17 ++++++-------
 arch/x86/kernel/hw_breakpoint.c      |  8 +++----
 arch/x86/kernel/kgdb.c               |  4 ++--
 arch/x86/kernel/process_32.c         |  6 ++---
 arch/x86/kernel/process_64.c         |  6 ++---
 arch/x86/kernel/traps.c              | 36 +++++++++++++++++-----------
 arch/x86/kvm/vmx/nested.c            |  2 +-
 arch/x86/kvm/vmx/vmx.c               |  6 ++---
 arch/x86/kvm/x86.c                   |  4 ++--
 14 files changed, 63 insertions(+), 51 deletions(-)

base-commit: 7cd9a11dd0c3d1dd225795ed1b5b53132888e7b5
-- 
2.49.0