xen/arm: arm64: Add emulation of Debug Data Transfer Registers

Currently, if user enables HVC_DCC config option in Linux, it invokes access
to debug data transfer registers (i.e. DBGDTRTX_EL0 on arm64, DBGDTRTXINT on
arm32). As these registers are not emulated, Xen injects an undefined
exception to the guest and Linux crashes.

To prevent this crash, introduce a partial emulation of DBGDTR[TR]X_EL0
(these registers share the same encoding) as RAZ/WI and MDCCSR_EL0 as TXfull.

Refer ARM DDI 0487J.a ID042523, D19.3.8, DBGDTRTX_EL0
"If TXfull is set to 1, set DTRRX and DTRTX to UNKNOWN".

Thus, any OS is expected to read MDCCSR_EL0 and check for TXfull before
using DBGDTRTX_EL0. Linux does it via hvc_dcc_init() ---> hvc_dcc_check(),
and returns -ENODEV in case TXfull bit is still set after writing a test
character. This way we prevent the guest from making use of HVC DCC as a
console.

Signed-off-by: Michal Orzel <michal.orzel@amd.com>
Signed-off-by: Ayan Kumar Halder <ayan.kumar.halder@amd.com>
Acked-by: Julien Grall <jgrall@amazon.com>

SUPPORT.md

@@ -111,6 +111,11 @@ that would normally trigger a fault injection.
 
 Only the following system registers are security supported:
 
+    MDCCSR_EL0
+    DBGDTR_EL0
+    DBGDTRTX_EL0
+    DBGDTRRX_EL0
+
 ### ARM Scalable Vector Extension (SVE/SVE2)
 
 Arm64 domains can use Scalable Vector Extension (SVE/SVE2).

xen/arch/arm/arm64/vsysreg.c

@@ -82,6 +82,7 @@ TVM_REG(CONTEXTIDR_EL1)
 void do_sysreg(struct cpu_user_regs *regs,
                const union hsr hsr)
 {
+    const struct hsr_sysreg sysreg = hsr.sysreg;
     int regidx = hsr.sysreg.reg;
     struct vcpu *v = current;
 
@@ -159,9 +160,6 @@ void do_sysreg(struct cpu_user_regs *regs,
      *
      * Unhandled:
      *    MDCCINT_EL1
-     *    DBGDTR_EL0
-     *    DBGDTRRX_EL0
-     *    DBGDTRTX_EL0
      *    OSDTRRX_EL1
      *    OSDTRTX_EL1
      *    OSECCR_EL1
@@ -171,12 +169,42 @@ void do_sysreg(struct cpu_user_regs *regs,
      */
     case HSR_SYSREG_MDSCR_EL1:
         return handle_raz_wi(regs, regidx, hsr.sysreg.read, hsr, 1);
+
+    /*
+     * Xen doesn't expose a real (or emulated) Debug Communications Channel
+     * (DCC) to a domain. Yet the Arm ARM implies this is not an optional
+     * feature. So some domains may start to probe it. For instance, the
+     * HVC_DCC driver in Linux (since f377775dc083 and at least up to v6.7),
+     * will try to write some characters and check if the transmit buffer
+     * has emptied.
+     */
     case HSR_SYSREG_MDCCSR_EL0:
         /*
+         * By setting TX status bit (only if partial emulation is enabled) to
+         * indicate the transmit buffer is full, we would hint the OS that the
+         * DCC is probably not working.
+         *
+         * Bit 29: TX full
+         *
          * Accessible at EL0 only if MDSCR_EL1.TDCC is set to 0. We emulate that
          * register as RAZ/WI above. So RO at both EL0 and EL1.
          */
-        return handle_ro_raz(regs, regidx, hsr.sysreg.read, hsr, 0);
+        return handle_ro_read_val(regs, regidx, hsr.sysreg.read, hsr, 0,
+                                  partial_emulation ? (1U << 29) : 0);
+
+    case HSR_SYSREG_DBGDTR_EL0:
+    /* DBGDTR[TR]X_EL0 share the same encoding */
+    case HSR_SYSREG_DBGDTRTX_EL0:
+        /*
+         * Emulate as RAZ/WI (only if partial emulation is enabled) to prevent
+         * injecting undefined exception.
+         * Accessible at EL0 only if MDSCR_EL1.TDCC is set to 0. We emulate that
+         * register as RAZ/WI.
+         */
+        if ( !partial_emulation )
+            goto fail;
+        return handle_raz_wi(regs, regidx, hsr.sysreg.read, hsr, 0);
+
     HSR_SYSREG_DBG_CASES(DBGBVR):
     HSR_SYSREG_DBG_CASES(DBGBCR):
     HSR_SYSREG_DBG_CASES(DBGWVR):
@@ -394,26 +422,25 @@ void do_sysreg(struct cpu_user_regs *regs,
      * And all other unknown registers.
      */
     default:
-        {
-            const struct hsr_sysreg sysreg = hsr.sysreg;
-
-            gdprintk(XENLOG_ERR,
-                     "%s %d, %d, c%d, c%d, %d %s x%d @ 0x%"PRIregister"\n",
-                     sysreg.read ? "mrs" : "msr",
-                     sysreg.op0, sysreg.op1,
-                     sysreg.crn, sysreg.crm,
-                     sysreg.op2,
-                     sysreg.read ? "=>" : "<=",
-                     sysreg.reg, regs->pc);
-            gdprintk(XENLOG_ERR,
-                     "unhandled 64-bit sysreg access %#"PRIregister"\n",
-                     hsr.bits & HSR_SYSREG_REGS_MASK);
-            inject_undef_exception(regs, hsr);
-            return;
-        }
+        goto fail;
     }
 
     regs->pc += 4;
+    return;
+
+ fail:
+    gdprintk(XENLOG_ERR,
+             "%s %d, %d, c%d, c%d, %d %s x%d @ 0x%"PRIregister"\n",
+             sysreg.read ? "mrs" : "msr",
+             sysreg.op0, sysreg.op1,
+             sysreg.crn, sysreg.crm,
+             sysreg.op2,
+             sysreg.read ? "=>" : "<=",
+             sysreg.reg, regs->pc);
+    gdprintk(XENLOG_ERR,
+             "unhandled 64-bit sysreg access %#"PRIregister"\n",
+             hsr.bits & HSR_SYSREG_REGS_MASK);
+    inject_undef_exception(regs, hsr);
 }
 
 /*

xen/arch/arm/include/asm/arm64/hsr.h

@@ -47,6 +47,9 @@
 #define HSR_SYSREG_OSDLR_EL1      HSR_SYSREG(2,0,c1,c3,4)
 #define HSR_SYSREG_DBGPRCR_EL1    HSR_SYSREG(2,0,c1,c4,4)
 #define HSR_SYSREG_MDCCSR_EL0     HSR_SYSREG(2,3,c0,c1,0)
+#define HSR_SYSREG_DBGDTR_EL0     HSR_SYSREG(2,3,c0,c4,0)
+#define HSR_SYSREG_DBGDTRTX_EL0   HSR_SYSREG(2,3,c0,c5,0)
+#define HSR_SYSREG_DBGDTRRX_EL0   HSR_SYSREG(2,3,c0,c5,0)
 
 #define HSR_SYSREG_DBGBVRn_EL1(n) HSR_SYSREG(2,0,c0,c##n,4)
 #define HSR_SYSREG_DBGBCRn_EL1(n) HSR_SYSREG(2,0,c0,c##n,5)