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

[Jiamei Xie <Jiamei.Xie@arm.com>]

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

Best wishes
Jiamei Xie



> -----Original Message-----
> From: Michal Orzel <michal.orzel@amd.com>
> Sent: Wednesday, November 9, 2022 3:40 PM
> To: Jiamei Xie <Jiamei.Xie@arm.com>; xen-devel@lists.xenproject.org
> Cc: Wei Chen <Wei.Chen@arm.com>; Bertrand Marquis
> <Bertrand.Marquis@arm.com>; julien@xen.org; sstabellini@kernel.org
> 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
> >