Re:

[Michal Orzel <michal.orzel@xxxxxxx>]

Hi Jimaei,

On 18/11/2022 03:00, Jiamei Xie wrote:
> 
> 
> Date: Thu, 17 Nov 2022 11:07:12 +0800
> Subject: [PATCH] xen/arm: vpl011: Make access to DMACR write-ignore
> 
> 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.
As discussed earlier, if we decide to improve vpl011 (for now only Stefano 
shared his opinion),
then we need to mark *all* the PL011 registers that are not part of SBSA ar 
RAZ/WI. So handling
DMACR and only for writes is not beneficial (it is only fixing current Linux 
issue, but what we
really want is to improve the code in general).

> 
> [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 | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/xen/arch/arm/vpl011.c b/xen/arch/arm/vpl011.c
> index 43522d48fd..80d00b3052 100644
> --- a/xen/arch/arm/vpl011.c
> +++ b/xen/arch/arm/vpl011.c
> @@ -463,6 +463,7 @@ static int vpl011_mmio_write(struct vcpu *v,
>      case FR:
>      case RIS:
>      case MIS:
> +    case DMACR:
>          goto write_ignore;
> 
>      case IMSC:
> --
> 2.25.1
> 
> 

~Michal