From: Michal Orzel <michal.or...@amd.com>
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.or...@amd.com>
Signed-off-by: Ayan Kumar Halder <ayan.kumar.hal...@amd.com>
---
Changes from
v1 :- 1. DBGDTR_EL0 does not emulate RXfull. This is to avoid giving the OS any
indication that the RX buffer is full and is waiting to be read.
2. In Arm32, DBGOSLSR is emulated. Also DBGDTRTXINT is emulated at EL0 only.
3. Fixed the commit message and inline code comments.
v2 :- 1. Split the patch into two (separate patches for arm64 and arm32).
2. Removed the "fail" label.
3. Fixed the commit message.
v3 :- 1. "HSR_SYSREG_MDCCSR_EL0" emulation differs based on whether
partial_emulation_enabled is true or not.
2. If partial_emulation_enabled is false, then access to HSR_SYSREG_DBGDTR_EL0,
HSR_SYSREG_DBGDTRTX_EL0 would lead to undefined exception.
xen/arch/arm/arm64/vsysreg.c | 28 ++++++++++++++++++++++++----
xen/arch/arm/include/asm/arm64/hsr.h | 3 +++
2 files changed, 27 insertions(+), 4 deletions(-)
diff --git a/xen/arch/arm/arm64/vsysreg.c b/xen/arch/arm/arm64/vsysreg.c
index b5d54c569b..94f0a6c384 100644
--- a/xen/arch/arm/arm64/vsysreg.c
+++ b/xen/arch/arm/arm64/vsysreg.c
@@ -159,9 +159,6 @@ void do_sysreg(struct cpu_user_regs *regs,
*
* Unhandled:
* MDCCINT_EL1
- * DBGDTR_EL0
- * DBGDTRRX_EL0
- * DBGDTRTX_EL0
* OSDTRRX_EL1
* OSDTRTX_EL1
* OSECCR_EL1
@@ -173,10 +170,32 @@ void do_sysreg(struct cpu_user_regs *regs,
return handle_raz_wi(regs, regidx, hsr.sysreg.read, hsr, 1);
case HSR_SYSREG_MDCCSR_EL0:
/*
+ * 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.
+ *
+ * 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:
+ 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):
@@ -394,6 +413,7 @@ void do_sysreg(struct cpu_user_regs *regs,
* And all other unknown registers.
*/
default:
+ fail:
{
const struct hsr_sysreg sysreg = hsr.sysreg;
diff --git a/xen/arch/arm/include/asm/arm64/hsr.h
b/xen/arch/arm/include/asm/arm64/hsr.h
index e691d41c17..1495ccddea 100644
--- a/xen/arch/arm/include/asm/arm64/hsr.h
+++ b/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)
--
2.25.1
