[PATCH 5/7] x86: Introduce x86_merge_dr6()

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

The current logic used to update %dr6 when injecting #DB is buggy.

The SDM/APM documention on %dr6 updates is far from ideal, but does at least
make clear that it's non-trivial.  The actual behaviour is to overwrite
B{0..3} and accumulate all other bits.

Introduce x86_merge_dr6() to perform the operaton properly.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
CC: Wei Liu <wl@xxxxxxx>
CC: Jinoh Kang <jinoh.kang.kr@xxxxxxxxx>

v2:
 * Extend commit message
---
 xen/arch/x86/debug.c                 | 20 ++++++++++++++++++++
 xen/arch/x86/include/asm/debugreg.h  |  7 +++++++
 xen/arch/x86/include/asm/x86-defns.h |  7 +++++++
 3 files changed, 34 insertions(+)

diff --git a/xen/arch/x86/debug.c b/xen/arch/x86/debug.c
index 127fe83021cd..bfcd83ea4d0b 100644
--- a/xen/arch/x86/debug.c
+++ b/xen/arch/x86/debug.c
@@ -3,6 +3,7 @@
  * Copyright (C) 2023 XenServer.
  */
 #include <xen/kernel.h>
+#include <xen/lib.h>
 
 #include <xen/lib/x86/cpu-policy.h>
 
@@ -28,6 +29,25 @@ unsigned int x86_adj_dr6_rsvd(const struct cpu_policy *p, 
unsigned int dr6)
     return dr6;
 }
 
+unsigned int x86_merge_dr6(const struct cpu_policy *p, unsigned int dr6,
+                           unsigned int new)
+{
+    /* Flip dr6 to have positive polarity. */
+    dr6 ^= X86_DR6_DEFAULT;
+
+    /* Sanity check that only known values are passed in. */
+    ASSERT(!(dr6 & ~X86_DR6_KNOWN_MASK));
+    ASSERT(!(new & ~X86_DR6_KNOWN_MASK));
+
+    /* Breakpoint matches are overridden.  All other bits accumulate. */
+    dr6 = (dr6 & ~X86_DR6_BP_MASK) | new;
+
+    /* Flip dr6 back to having default polarity. */
+    dr6 ^= X86_DR6_DEFAULT;
+
+    return x86_adj_dr6_rsvd(p, dr6);
+}
+
 unsigned int x86_adj_dr7_rsvd(const struct cpu_policy *p, unsigned int dr7)
 {
     unsigned int zeros = X86_DR7_ZEROS;
diff --git a/xen/arch/x86/include/asm/debugreg.h 
b/xen/arch/x86/include/asm/debugreg.h
index 39ba312b84ee..e98a9ce977fa 100644
--- a/xen/arch/x86/include/asm/debugreg.h
+++ b/xen/arch/x86/include/asm/debugreg.h
@@ -89,4 +89,11 @@ struct cpu_policy;
 unsigned int x86_adj_dr6_rsvd(const struct cpu_policy *p, unsigned int dr6);
 unsigned int x86_adj_dr7_rsvd(const struct cpu_policy *p, unsigned int dr7);
 
+/*
+ * Merge new bits into dr6.  'new' is always given in positive polarity,
+ * matching the Intel VMCS PENDING_DBG semantics.
+ */
+unsigned int x86_merge_dr6(const struct cpu_policy *p, unsigned int dr6,
+                           unsigned int new);
+
 #endif /* _X86_DEBUGREG_H */
diff --git a/xen/arch/x86/include/asm/x86-defns.h 
b/xen/arch/x86/include/asm/x86-defns.h
index 5838631ef634..edfecc89bd08 100644
--- a/xen/arch/x86/include/asm/x86-defns.h
+++ b/xen/arch/x86/include/asm/x86-defns.h
@@ -116,6 +116,13 @@
 #define X86_DR6_BT              (_AC(1, UL) << 15)   /* Task switch            
     */
 #define X86_DR6_RTM             (_AC(1, UL) << 16)   /* #DB/#BP in RTM region 
(INV) */
 
+#define X86_DR6_BP_MASK                                 \
+    (X86_DR6_B0 | X86_DR6_B1 | X86_DR6_B2 | X86_DR6_B3)
+
+#define X86_DR6_KNOWN_MASK                                      \
+    (X86_DR6_BP_MASK | X86_DR6_BLD | X86_DR6_BD | X86_DR6_BS |  \
+     X86_DR6_BT | X86_DR6_RTM)
+
 #define X86_DR6_ZEROS           _AC(0x00001000, UL)  /* %dr6 bits forced to 0  
     */
 #define X86_DR6_DEFAULT         _AC(0xffff0ff0, UL)  /* Default %dr6 value     
     */
 
-- 
2.30.2