Re: [PATCH v3] hw/arm/virt: Avoid unexpected warning from Linux guest on host with Fujitsu CPUs

[Jonathan Cameron via <qemu-arm@nongnu.org>]

On Tue, 11 Jun 2024 22:05:06 -0400
Zhenyu Zhang <zhenyzha@redhat.com> wrote:

> Multiple warning messages and corresponding backtraces are observed when Linux
> guest is booted on the host with Fujitsu CPUs. One of them is shown as below.
> 
> [    0.032443] ------------[ cut here ]------------
> [    0.032446] uart-pl011 9000000.pl011: ARCH_DMA_MINALIGN smaller than
> CTR_EL0.CWG (128 < 256)
> [    0.032454] WARNING: CPU: 0 PID: 1 at arch/arm64/mm/dma-mapping.c:54
> arch_setup_dma_ops+0xbc/0xcc
> [    0.032470] Modules linked in:
> [    0.032475] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 5.14.0-452.el9.aarch64
> [    0.032481] Hardware name: linux,dummy-virt (DT)
> [    0.032484] pstate: 60400005 (nZCv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
> [    0.032490] pc : arch_setup_dma_ops+0xbc/0xcc
> [    0.032496] lr : arch_setup_dma_ops+0xbc/0xcc
> [    0.032501] sp : ffff80008003b860
> [    0.032503] x29: ffff80008003b860 x28: 0000000000000000 x27: ffffaae4b949049c
> [    0.032510] x26: 0000000000000000 x25: 0000000000000000 x24: 0000000000000000
> [    0.032517] x23: 0000000000000100 x22: 0000000000000000 x21: 0000000000000000
> [    0.032523] x20: 0000000100000000 x19: ffff2f06c02ea400 x18: ffffffffffffffff
> [    0.032529] x17: 00000000208a5f76 x16: 000000006589dbcb x15: ffffaae4ba071c89
> [    0.032535] x14: 0000000000000000 x13: ffffaae4ba071c84 x12: 455f525443206e61
> [    0.032541] x11: 68742072656c6c61 x10: 0000000000000029 x9 : ffffaae4b7d21da4
> [    0.032547] x8 : 0000000000000029 x7 : 4c414e494d5f414d x6 : 0000000000000029
> [    0.032553] x5 : 000000000000000f x4 : ffffaae4b9617a00 x3 : 0000000000000001
> [    0.032558] x2 : 0000000000000000 x1 : 0000000000000000 x0 : ffff2f06c029be40
> [    0.032564] Call trace:
> [    0.032566]  arch_setup_dma_ops+0xbc/0xcc
> [    0.032572]  of_dma_configure_id+0x138/0x300
> [    0.032591]  amba_dma_configure+0x34/0xc0
> [    0.032600]  really_probe+0x78/0x3dc
> [    0.032614]  __driver_probe_device+0x108/0x160
> [    0.032619]  driver_probe_device+0x44/0x114
> [    0.032624]  __device_attach_driver+0xb8/0x14c
> [    0.032629]  bus_for_each_drv+0x88/0xe4
> [    0.032634]  __device_attach+0xb0/0x1e0
> [    0.032638]  device_initial_probe+0x18/0x20
> [    0.032643]  bus_probe_device+0xa8/0xb0
> [    0.032648]  device_add+0x4b4/0x6c0
> [    0.032652]  amba_device_try_add.part.0+0x48/0x360
> [    0.032657]  amba_device_add+0x104/0x144
> [    0.032662]  of_amba_device_create.isra.0+0x100/0x1c4
> [    0.032666]  of_platform_bus_create+0x294/0x35c
> [    0.032669]  of_platform_populate+0x5c/0x150
> [    0.032672]  of_platform_default_populate_init+0xd0/0xec
> [    0.032697]  do_one_initcall+0x4c/0x2e0
> [    0.032701]  do_initcalls+0x100/0x13c
> [    0.032707]  kernel_init_freeable+0x1c8/0x21c
> [    0.032712]  kernel_init+0x28/0x140
> [    0.032731]  ret_from_fork+0x10/0x20
> [    0.032735] ---[ end trace 0000000000000000 ]---
> 
> In Linux, a check is applied to every device which is exposed through
> device-tree node. The warning message is raised when the device isn't
> DMA coherent and the cache line size is larger than ARCH_DMA_MINALIGN
> (128 bytes). The cache line is sorted from CTR_EL0[CWG], which corresponds
> to 256 bytes on the guest CPUs. The DMA coherent capability is claimed
> through 'dma-coherent' in their device-tree nodes or parent nodes.
> 
> Fix the issue by adding 'dma-coherent' property to the device-tree root
> node, meaning all devices are capable of DMA coherent by default.
> 
> Signed-off-by: Zhenyu Zhang <zhenyzha@redhat.com>
Aligns with earlier discussion so...

Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

> ---
> v3: Add comments explaining why we add 'dma-coherent' property (Peter)
> ---
>  hw/arm/virt.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/hw/arm/virt.c b/hw/arm/virt.c
> index 3c93c0c0a6..3cefac6d43 100644
> --- a/hw/arm/virt.c
> +++ b/hw/arm/virt.c
> @@ -271,6 +271,17 @@ static void create_fdt(VirtMachineState *vms)
>      qemu_fdt_setprop_cell(fdt, "/", "#size-cells", 0x2);
>      qemu_fdt_setprop_string(fdt, "/", "model", "linux,dummy-virt");
>  
> +    /*
> +     * For QEMU, all DMA is coherent. Advertising this in the root node
> +     * has two benefits:
> +     *
> +     * - It avoids potential bugs where we forget to mark a DMA
> +     *   capable device as being dma-coherent
> +     * - It avoids spurious warnings from the Linux kernel about
> +     *   devices which can't do DMA at all
> +     */
> +    qemu_fdt_setprop(fdt, "/", "dma-coherent", NULL, 0);
> +
>      /* /chosen must exist for load_dtb to fill in necessary properties later */
>      qemu_fdt_add_subnode(fdt, "/chosen");
>      if (vms->dtb_randomness) {