Re: [PATCH v4 1/2] xen/arm: vpl011: emulate non-SBSA registers as WI/RAZ

[Michal Orzel <michal.orzel@xxxxxxx>]

On 01/12/2022 09:42, Michal Orzel wrote:
> 
> 
> Hi Jiamei,
> 
> On 01/12/2022 09:03, Jiamei Xie wrote:
>>
>>
>> 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 non-SBSA registers
>> RAZ/WI as an improvement.
>>
>> [1] 
>> https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fdeveloper.arm.com%2Fdocumentation%2Fden0094%2Fc%2F%3Flang%3Den&data=05%7C01%7Cmichal.orzel%40amd.com%7C9345e5fe73c04ece2bbc08dad377f56e%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C638054809425877261%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000%7C%7C%7C&sdata=%2FTuXUhLO21PljT1o4tTEtyU0hXgDLEUjp3t0X1AnXXk%3D&reserved=0
>> [2] 
>> https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Flore.kernel.org%2Fxen-devel%2Falpine.DEB.2.22.394.2211161552420.4020%40ubuntu-linux-20-04-desktop%2F&data=05%7C01%7Cmichal.orzel%40amd.com%7C9345e5fe73c04ece2bbc08dad377f56e%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C638054809425877261%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000%7C%7C%7C&sdata=c4qOLzf4w32%2BVkt49umv%2Bu3LVP3TVsQ8iBFLoLpRclg%3D&reserved=0
>>
>> Signed-off-by: Jiamei Xie <jiamei.xie@xxxxxxx>
> The patch looks good, so:
> Reviewed-by: Michal Orzel <michal.orzel@xxxxxxx>
> 
> However, because your series is about vpl011 refinement, I spotted two things
> (this does not necessarily needs to be done by you).
> 
>> ---
>> v3 -> v4
>> - remove the size check for unknown registers in the SBSA UART
>> - remove lock in read_as_zero
>> v2 -> v3
>> - emulate non-SBSA registers as WI/RAZ in default case
>> - update commit message
>> v1 -> v2
>> - print a message using XENLOG_G_DEBUG when it's write-ignore
>> ---
>>  xen/arch/arm/vpl011.c | 8 ++++++--
>>  1 file changed, 6 insertions(+), 2 deletions(-)
>>
>> diff --git a/xen/arch/arm/vpl011.c b/xen/arch/arm/vpl011.c
>> index 43522d48fd..f4a5621fab 100644
>> --- a/xen/arch/arm/vpl011.c
>> +++ b/xen/arch/arm/vpl011.c
>> @@ -414,11 +414,15 @@ static int vpl011_mmio_read(struct vcpu *v,
>>      default:
>>          gprintk(XENLOG_ERR, "vpl011: unhandled read r%d offset %#08x\n",
> This is an emulated UART MMIO handler, so instead XENLOG_ERR, we should use 
> XENLOG_G_ERR
> to indicate gust error and not Xen error.
> 
>>                  dabt.reg, vpl011_reg);
>> -        return 0;
>> +        goto read_as_zero;
>>      }
>>
>>      return 1;
> This return statement is unreachable.
Forget about this one. I can see you fixed that in the second patch.

~Michal