Re: Xen Arm vpl011 UART will cause segmentation fault in Linux guest

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Wed, 9 Nov 2022, Michal Orzel wrote:
> Hi Jiamei,
> 
> On 09/11/2022 09:25, Jiamei Xie wrote:
> > 
> > 
> > Hi Michal，
> > 
> > Below log can be got when stating the linux guest. It says 9c09 is sbsa. 
> > And 9c09 is also output
> >  in bootlogd error message:
> > Serial: AMBA PL011 UART driver
> > 9c0b0000.uart: ttyAMA0 at MMIO 0x9c0b0000 (irq = 12, base_baud = 0) is a 
> > PL011 rev2
> > printk: console [ttyAMA0] enabled
> > 9c090000.sbsa-uart: ttyAMA1 at MMIO 0x9c090000 (irq = 15, base_baud = 0) is 
> > a SBSA
> > 
> 
> Xen behavior is correct and this would be Linux fault to try to write to 
> DMACR for SBSA UART device.
> DMACR is just an example. If you try to program e.g. the baudrate (through 
> LCR) for VPL011 it will
> also result in injecting abort into the guest. Should Xen support it? No. The 
> reason why is that
> it is not spec compliant operation. SBSA specification directly specifies 
> what registers are exposed.
> If Linux tries to write to some of the none-spec compliant registers - it is 
> its fault.

Yeah, we need to fix Linux.

FYI this is not the first bug in Linux affecting the sbsa-uart driver:
the issue is that the pl011 driver and the sbsa-uart driver share the
same code in Linux so it happens sometimes that a pl011-only feature
creeps into the sbsa-uart driver by mistake.


> >> -----Original Message-----
> >> From: Michal Orzel <michal.orzel@xxxxxxx>
> >> Sent: Wednesday, November 9, 2022 3:40 PM
> >> To: Jiamei Xie <Jiamei.Xie@xxxxxxx>; xen-devel@xxxxxxxxxxxxxxxxxxxx
> >> Cc: Wei Chen <Wei.Chen@xxxxxxx>; Bertrand Marquis
> >> <Bertrand.Marquis@xxxxxxx>; julien@xxxxxxx; sstabellini@xxxxxxxxxx
> >> Subject: Re: Xen Arm vpl011 UART will cause segmentation fault in Linux
> >> guest
> >>
> >> Hi Jiamei,
> >>
> >> On 09/11/2022 08:20, Jiamei Xie wrote:
> >>>
> >>>
> >>> Hi all,
> >>>
> >>> When the guest kernel enables DMA engine with
> >> "CONFIG_DMA_ENGINE=y", Linux AMBA PL011 driver will access PL011
> >> DMACR register. But this register have not been supported by vpl011 of Xen.
> >> Xen will inject a data abort into guest, this will cause segmentation 
> >> fault of
> >> guest with the below message:
> >> I am quite confused.
> >> VPL011 implements SBSA UART which only implements some subset of PL011
> >> operations (SBSA UART is not PL011).
> >> According to spec (SBSA ver. 6.0), the SBSA_UART does not support DMA
> >> features so Xen code is fine.
> >> When Xen exposes vpl011 device to a guest, this device has "arm,sbsa-uart"
> >> compatible and not "uart-pl011".
> >> Linux driver "amba-pl011.c" should see this compatible and assign proper
> >> operations (sbsa_uart_pops instead of amba_pl011_pops) that do not enable
> >> DMA.
> >> Maybe the issue is with your configuration?
> >>
> >> ~Michal
> >>
> >>> 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
> >>> Modules linked in:
> >>> CPU: 0 PID: 132 Comm: bootlogd Not tainted 5.15.44-yocto-standard #1
> >>> pstate: 604000c5 (nZCv daIF +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
> >>> pc : pl011_stop_rx+0x70/0x80
> >>> lr : uart_tty_port_shutdown+0x44/0x110
> >>> sp : ffffffc00999bba0
> >>> x29: ffffffc00999bba0 x28: ffffff80234ac380 x27: ffffff8022f5d000
> >>> x26: 0000000000000000 x25: 0000000045585401 x24: 0000000000000000
> >>> x23: ffffff8021ba4660 x22: 0000000000000001 x21: ffffff8021a0e2a0
> >>> x20: ffffff802198f880 x19: ffffff8021a0e1a0 x18: 0000000000000000
> >>> x17: 0000000000000000 x16: 0000000000000000 x15: 0000000000000000
> >>> x14: 0000000000000000 x13: 0000000000000000 x12: 0000000000000000
> >>> x11: 0000000000000000 x10: 0000000000000000 x9 : ffffffc00871ba14
> >>> x8 : ffffffc0099de260 x7 : ffffff8021a0e318 x6 : 0000000000000003
> >>> x5 : ffffffc009315f20 x4 : ffffffc00944d038 x3 : 0000000000000000
> >>> x2 : ffffffc00944d048 x1 : 0000000000000000 x0 : 0000000000000048
> >>> 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
> >>> In Xen, vpl011_mmio_write doesn't handle DMACR . And kernel doesn't
> >> check if pl011_write executes sucessfully in pl011_dma_rx_stop . So such
> >> segmentation fault occurs.
> >>> static inline void pl011_dma_rx_stop(struct uart_amba_port *uap)
> >>> {
> >>>         /* FIXME.  Just disable the DMA enable */
> >>>         uap->dmacr &= ~UART011_RXDMAE;
> >>>         pl011_write(uap->dmacr, uap, REG_DMACR);
> >>> }
> >>>
> >>> I think we should prevent such segmentation fault. We have checked the
> >> PL011 spec, it seems there is not any register bit can indicate DMA support
> >> status of PL011. We might have two options:
> >>> 1. Option#1 is to add DMA support for vpl011, but this is not trivial.
> >>> 2. Option#2 is to ignore the write to DMACR, and return 0 for DMACR read
> >> in vpl011. But this option need co-work with kernel, because current Linux
> >> PL011 driver assume the write operation will never be failed, and will not
> >> fallback to no-DMA mode, when Xen return 0 for DMA enabled bit in DMACR.
> >>>
> >>> How do you think about it?  Any suggestion about it is welcome. Thanks.
> >>>
> >>> Best wishes
> >>> Jiamei Xie
> >>>
>