[Xen-devel] [PATCH 3/4] xen/arm: Implement a dummy debug monitor for ARM32

[Julien Grall <julien.grall@xxxxxxxxxx>]

XSA-93 (commit 0b18220 "xen/arm: Don't let guess access to Debug and Performance
Monitors registers") disable Debug Registers access.

When CONFIG_PERF_EVENTS is enabled in the Linux Kernel, it will try to
initialize the debug monitors. If an error occured Linux won't use this
feature.

The implementation made Xen expose a minimal set of registers which let think
the guest (i.e.) thinks HW debug won't work.

Signed-off-by: Julien Grall <julien.grall@xxxxxxxxxx>

---
    This commit is candidate for backporting on Xen 4.4. Distribution may
    enable HW DEBUG (e.g. Linaro Ubuntu image), therefore Linux will try
    to initialize it.
---
 xen/arch/arm/traps.c         |   77 ++++++++++++++++++++++++++++++++++++++++--
 xen/include/asm-arm/cpregs.h |   14 ++++++++
 2 files changed, 89 insertions(+), 2 deletions(-)

diff --git a/xen/arch/arm/traps.c b/xen/arch/arm/traps.c
index 319bbe9..1f61e6e 100644
--- a/xen/arch/arm/traps.c
+++ b/xen/arch/arm/traps.c
@@ -1468,7 +1468,76 @@ static void do_cp15_64(struct cpu_user_regs *regs,
     advance_pc(regs, hsr);
 }
 
-static void do_cp14(struct cpu_user_regs *regs, union hsr hsr)
+static void do_cp14_32(struct cpu_user_regs *regs, union hsr hsr)
+{
+    struct hsr_cp32 cp32 = hsr.cp32;
+    uint32_t *r = (uint32_t *)select_user_reg(regs, cp32.reg);
+    struct domain *d = current->domain;
+
+    if ( !check_conditional_instr(regs, hsr) )
+    {
+        advance_pc(regs, hsr);
+        return;
+    }
+
+    switch ( hsr.bits & HSR_CP32_REGS_MASK )
+    {
+    case HSR_CPREG32(DBGDIDR):
+
+        /* Read-only register */
+        if ( !cp32.read )
+            goto bad_cp;
+
+        /* Implement the minimum requirements:
+         *  - Number of watchpoints: 1
+         *  - Number of breakpoints: 2
+         *  - Version: ARMv7 v7.1
+         *  - Variant and Revision bits match MDIR
+         */
+        *r = (1 << 24) | (5 << 16);
+        *r |= ((d->arch.vpidr >> 20) & 0xf) | (d->arch.vpidr & 0xf);
+        break;
+
+    case HSR_CPREG32(DBGDSCRINT):
+    case HSR_CPREG32(DBGDSCREXT):
+        /* Implement debug status and control register as RAZ/WI.
+         * The OS won't use Hardware debug if MDBGen not set
+         */
+        if ( cp32.read )
+           *r = 0;
+        break;
+    case HSR_CPREG32(DBGVCR):
+    case HSR_CPREG32(DBGOSLAR):
+    case HSR_CPREG32(DBGBVR0):
+    case HSR_CPREG32(DBGCR0):
+    case HSR_CPREG32(DBGWVR0):
+    case HSR_CPREG32(DBGWCR0):
+    case HSR_CPREG32(DBGBVR1):
+    case HSR_CPREG32(DBGCR1):
+    case HSR_CPREG32(DBGOSDLR):
+        /* RAZ/WI */
+        if ( cp32.read )
+            *r = 0;
+        break;
+
+    default:
+bad_cp:
+#ifndef NDEBUG
+        gdprintk(XENLOG_ERR,
+                 "%s p14, %d, r%d, cr%d, cr%d, %d @ 0x%"PRIregister"\n",
+                  cp32.read ? "mrc" : "mcr",
+                  cp32.op1, cp32.reg, cp32.crn, cp32.crm, cp32.op2, regs->pc);
+        gdprintk(XENLOG_ERR, "unhandled 32-bit cp14 access %#x\n",
+                 hsr.bits & HSR_CP32_REGS_MASK);
+#endif
+        inject_undef32_exception(regs);
+        return;
+    }
+
+    advance_pc(regs, hsr);
+}
+
+static void do_cp14_dbg(struct cpu_user_regs *regs, union hsr hsr)
 {
     if ( !check_conditional_instr(regs, hsr) )
     {
@@ -1720,10 +1789,14 @@ asmlinkage void do_trap_hypervisor(struct cpu_user_regs 
*regs)
         do_cp15_64(regs, hsr);
         break;
     case HSR_EC_CP14_32:
+        if ( !is_32bit_domain(current->domain) )
+            goto bad_trap;
+        do_cp14_32(regs, hsr);
+        break;
     case HSR_EC_CP14_DBG:
         if ( !is_32bit_domain(current->domain) )
             goto bad_trap;
-        do_cp14(regs, hsr);
+        do_cp14_dbg(regs, hsr);
         break;
     case HSR_EC_CP:
         if ( !is_32bit_domain(current->domain) )
diff --git a/xen/include/asm-arm/cpregs.h b/xen/include/asm-arm/cpregs.h
index f44e3b5..306e506 100644
--- a/xen/include/asm-arm/cpregs.h
+++ b/xen/include/asm-arm/cpregs.h
@@ -71,6 +71,20 @@
 
 /* Coprocessor 14 */
 
+/* CP14 0: Debug Register interface */
+#define DBGDIDR         p14,0,c0,c0,0   /* Debug ID Register */
+#define DBGDSCRINT      p14,0,c0,c1,0   /* Debug Status and Control Internal */
+#define DBGDSCREXT      p14,0,c0,c2,2   /* Debug Status and Control External */
+#define DBGVCR          p14,0,c0,c7,0   /* Vector Catch */
+#define DBGBVR0         p14,0,c0,c0,4   /* Breakpoint Value 0 */
+#define DBGCR0          p14,0,c0,c0,5   /* Breakpoint Control 0 */
+#define DBGWVR0         p14,0,c0,c0,6   /* Watchpoint Value 0 */
+#define DBGWCR0         p14,0,c0,c0,7   /* Watchpoint Control 0 */
+#define DBGBVR1         p14,0,c0,c1,4   /* Breakpoint Value 1 */
+#define DBGCR1          p14,0,c0,c1,5   /* Breakpoint Control 1 */
+#define DBGOSLAR        p14,0,c1,c0,4   /* OS Lock Access */
+#define DBGOSDLR        p14,0,c1,c3,4   /* OS Double Lock */
+
 /* CP14 CR0: */
 #define TEECR           p14,6,c0,c0,0   /* ThumbEE Configuration Register */
 
-- 
1.7.10.4


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