[PATCH 5/5] x86/pv: Rewrite %dr6 handling

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

All #DB exceptions result in an update of %dr6, but this isn't handled
properly by Xen for any guest type.

To start with, add a new pending_dbg field to x86_event, sharing storage with
cr2, and using the Intel VMCS PENDING_DBG semantics.  Also introduce a
pv_inject_DB() wrapper use this field nicely.

Remove all ad-hoc dr6 handling, leaving it to pv_inject_event() in most cases
and using the new x86_merge_dr6() helper.

In do_debug(), adjust dr6 manually only when a debugger is attached.  This
maintains the old behaviour.

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>
---
 xen/arch/x86/include/asm/domain.h      | 12 ++++++++++++
 xen/arch/x86/pv/emul-priv-op.c         |  5 +----
 xen/arch/x86/pv/emulate.c              |  6 ++----
 xen/arch/x86/pv/ro-page-fault.c        |  4 ++--
 xen/arch/x86/pv/traps.c                | 17 +++++++++++++----
 xen/arch/x86/traps.c                   | 12 +++++++-----
 xen/arch/x86/x86_emulate/x86_emulate.h |  5 ++++-
 7 files changed, 41 insertions(+), 20 deletions(-)

diff --git a/xen/arch/x86/include/asm/domain.h 
b/xen/arch/x86/include/asm/domain.h
index c2d9fc333be5..5bf488437ce1 100644
--- a/xen/arch/x86/include/asm/domain.h
+++ b/xen/arch/x86/include/asm/domain.h
@@ -729,6 +729,18 @@ static inline void pv_inject_hw_exception(unsigned int 
vector, int errcode)
     pv_inject_event(&event);
 }
 
+static inline void pv_inject_DB(unsigned long pending_dbg)
+{
+    struct x86_event event = {
+        .vector      = X86_EXC_DB,
+        .type        = X86_EVENTTYPE_HW_EXCEPTION,
+        .error_code  = X86_EVENT_NO_EC,
+        .pending_dbg = pending_dbg,
+    };
+
+    pv_inject_event(&event);
+}
+
 static inline void pv_inject_page_fault(int errcode, unsigned long cr2)
 {
     const struct x86_event event = {
diff --git a/xen/arch/x86/pv/emul-priv-op.c b/xen/arch/x86/pv/emul-priv-op.c
index 6963db35c960..437172ee0fc3 100644
--- a/xen/arch/x86/pv/emul-priv-op.c
+++ b/xen/arch/x86/pv/emul-priv-op.c
@@ -1365,10 +1365,7 @@ int pv_emulate_privileged_op(struct cpu_user_regs *regs)
         ASSERT(!curr->arch.pv.trap_bounce.flags);
 
         if ( ctxt.ctxt.retire.pending_dbg )
-        {
-            curr->arch.dr6 |= ctxt.ctxt.retire.pending_dbg | 
DR_STATUS_RESERVED_ONE;
-            pv_inject_hw_exception(X86_EXC_DB, X86_EVENT_NO_EC);
-        }
+            pv_inject_DB(ctxt.ctxt.retire.pending_dbg);
 
         /* fall through */
     case X86EMUL_RETRY:
diff --git a/xen/arch/x86/pv/emulate.c b/xen/arch/x86/pv/emulate.c
index e7a1c0a2cc4f..e522e58533f1 100644
--- a/xen/arch/x86/pv/emulate.c
+++ b/xen/arch/x86/pv/emulate.c
@@ -71,11 +71,9 @@ void pv_emul_instruction_done(struct cpu_user_regs *regs, 
unsigned long rip)
 {
     regs->rip = rip;
     regs->eflags &= ~X86_EFLAGS_RF;
+
     if ( regs->eflags & X86_EFLAGS_TF )
-    {
-        current->arch.dr6 |= DR_STEP | DR_STATUS_RESERVED_ONE;
-        pv_inject_hw_exception(X86_EXC_DB, X86_EVENT_NO_EC);
-    }
+        pv_inject_DB(X86_DR6_BS);
 }
 
 uint64_t pv_get_reg(struct vcpu *v, unsigned int reg)
diff --git a/xen/arch/x86/pv/ro-page-fault.c b/xen/arch/x86/pv/ro-page-fault.c
index cad28ef928ad..f6bb33556e72 100644
--- a/xen/arch/x86/pv/ro-page-fault.c
+++ b/xen/arch/x86/pv/ro-page-fault.c
@@ -389,8 +389,8 @@ int pv_ro_page_fault(unsigned long addr, struct 
cpu_user_regs *regs)
 
         /* Fallthrough */
     case X86EMUL_OKAY:
-        if ( ctxt.retire.singlestep )
-            pv_inject_hw_exception(X86_EXC_DB, X86_EVENT_NO_EC);
+        if ( ctxt.retire.pending_dbg )
+            pv_inject_DB(ctxt.retire.pending_dbg);
 
         /* Fallthrough */
     case X86EMUL_RETRY:
diff --git a/xen/arch/x86/pv/traps.c b/xen/arch/x86/pv/traps.c
index 74f333da7e1c..553b04bca956 100644
--- a/xen/arch/x86/pv/traps.c
+++ b/xen/arch/x86/pv/traps.c
@@ -13,6 +13,7 @@
 #include <xen/softirq.h>
 
 #include <asm/pv/trace.h>
+#include <asm/debugreg.h>
 #include <asm/shared.h>
 #include <asm/traps.h>
 #include <irq_vectors.h>
@@ -50,9 +51,9 @@ void pv_inject_event(const struct x86_event *event)
     tb->cs    = ti->cs;
     tb->eip   = ti->address;
 
-    if ( event->type == X86_EVENTTYPE_HW_EXCEPTION &&
-         vector == X86_EXC_PF )
+    switch ( vector | -(event->type == X86_EVENTTYPE_SW_INTERRUPT) )
     {
+    case X86_EXC_PF:
         curr->arch.pv.ctrlreg[2] = event->cr2;
         arch_set_cr2(curr, event->cr2);
 
@@ -62,9 +63,17 @@ void pv_inject_event(const struct x86_event *event)
             error_code |= PFEC_user_mode;
 
         trace_pv_page_fault(event->cr2, error_code);
-    }
-    else
+        break;
+
+    case X86_EXC_DB:
+        curr->arch.dr6 = x86_merge_dr6(curr->domain->arch.cpu_policy,
+                                       curr->arch.dr6, event->pending_dbg);
+        /* Fallthrough */
+
+    default:
         trace_pv_trap(vector, regs->rip, use_error_code, error_code);
+        break;
+    }
 
     if ( use_error_code )
     {
diff --git a/xen/arch/x86/traps.c b/xen/arch/x86/traps.c
index dead728ce329..ae5d73abf557 100644
--- a/xen/arch/x86/traps.c
+++ b/xen/arch/x86/traps.c
@@ -1887,7 +1887,7 @@ void do_device_not_available(struct cpu_user_regs *regs)
 /* SAF-1-safe */
 void do_debug(struct cpu_user_regs *regs)
 {
-    unsigned long dr6;
+    unsigned long dr6, pending_dbg;
     struct vcpu *v = current;
 
     /* Stash dr6 as early as possible. */
@@ -1997,17 +1997,19 @@ void do_debug(struct cpu_user_regs *regs)
         return;
     }
 
-    /* Save debug status register where guest OS can peek at it */
-    v->arch.dr6 |= (dr6 & ~X86_DR6_DEFAULT);
-    v->arch.dr6 &= (dr6 | ~X86_DR6_DEFAULT);
+    /* Flip dr6 to have positive polarity. */
+    pending_dbg = dr6 ^ X86_DR6_DEFAULT;
 
     if ( guest_kernel_mode(v, regs) && v->domain->debugger_attached )
     {
+        /* Save debug status register where gdbsx can peek at it */
+        v->arch.dr6 = x86_merge_dr6(v->domain->arch.cpu_policy,
+                                    v->arch.dr6, pending_dbg);
         domain_pause_for_debugger();
         return;
     }
 
-    pv_inject_hw_exception(X86_EXC_DB, X86_EVENT_NO_EC);
+    pv_inject_DB(pending_dbg);
 }
 
 /* SAF-1-safe */
diff --git a/xen/arch/x86/x86_emulate/x86_emulate.h 
b/xen/arch/x86/x86_emulate/x86_emulate.h
index f0e74d23c378..81f99dfaa02f 100644
--- a/xen/arch/x86/x86_emulate/x86_emulate.h
+++ b/xen/arch/x86/x86_emulate/x86_emulate.h
@@ -78,7 +78,10 @@ struct x86_event {
     uint8_t       type;         /* X86_EVENTTYPE_* */
     uint8_t       insn_len;     /* Instruction length */
     int32_t       error_code;   /* X86_EVENT_NO_EC if n/a */
-    unsigned long cr2;          /* Only for X86_EXC_PF h/w exception */
+    union {
+        unsigned long cr2;         /* #PF */
+        unsigned long pending_dbg; /* #DB (new DR6 bits, positive polarity) */
+    };
 };
 
 /*
-- 
2.30.2