[PATCH v2] xen/arm: vpl011: Make access to DMACR write-ignore

[Jiamei Xie <jiamei.xie@xxxxxxx>]

When the guest kernel enables DMA engine with "CONFIG_DMA_ENGINE=y",
Linux SBSA PL011 driver will access PL011 DMACR register in some
functions. As chapter "B Generic UART" in "ARM Server Base System
Architecture"[1] documentation describes, SBSA UART doesn't support
DMA. In current code, when the kernel tries to access DMACR register,
Xen will inject a data abort:
Unhandled fault at 0xffffffc00944d048
Mem abort info:
  ESR = 0x96000000
  EC = 0x25: DABT (current EL), IL = 32 bits
  SET = 0, FnV = 0
  EA = 0, S1PTW = 0
  FSC = 0x00: ttbr address size fault
Data abort info:
  ISV = 0, ISS = 0x00000000
  CM = 0, WnR = 0
swapper pgtable: 4k pages, 39-bit VAs, pgdp=0000000020e2e000
[ffffffc00944d048] pgd=100000003ffff803, p4d=100000003ffff803, 
pud=100000003ffff803, pmd=100000003fffa803, pte=006800009c090f13
Internal error: ttbr address size fault: 96000000 [#1] PREEMPT SMP
...
Call trace:
 pl011_stop_rx+0x70/0x80
 tty_port_shutdown+0x7c/0xb4
 tty_port_close+0x60/0xcc
 uart_close+0x34/0x8c
 tty_release+0x144/0x4c0
 __fput+0x78/0x220
 ____fput+0x1c/0x30
 task_work_run+0x88/0xc0
 do_notify_resume+0x8d0/0x123c
 el0_svc+0xa8/0xc0
 el0t_64_sync_handler+0xa4/0x130
 el0t_64_sync+0x1a0/0x1a4
Code: b9000083 b901f001 794038a0 8b000042 (b9000041)
---[ end trace 83dd93df15c3216f ]---
note: bootlogd[132] exited with preempt_count 1
/etc/rcS.d/S07bootlogd: line 47: 132 Segmentation fault start-stop-daemon

As discussed in [2], this commit makes the access to DMACR register
write-ignore as an improvement.

[1] https://developer.arm.com/documentation/den0094/c/?lang=en
[2] 
https://lore.kernel.org/xen-devel/alpine.DEB.2.22.394.2211161552420.4020@ubuntu-linux-20-04-desktop/

Signed-off-by: Jiamei Xie <jiamei.xie@xxxxxxx>
---
 xen/arch/arm/vpl011.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/xen/arch/arm/vpl011.c b/xen/arch/arm/vpl011.c
index 43522d48fd..e97fe3ebe7 100644
--- a/xen/arch/arm/vpl011.c
+++ b/xen/arch/arm/vpl011.c
@@ -463,6 +463,10 @@ static int vpl011_mmio_write(struct vcpu *v,
     case FR:
     case RIS:
     case MIS:
+    case DMACR:
+        printk(XENLOG_G_DEBUG
+               "vpl011: WI on register offset %#08x\n",
+               vpl011_reg);
         goto write_ignore;
 
     case IMSC:
-- 
2.25.1