linux-kernel - Re: [RFC PATCH 6/6] arm/xen: Assign xen-virtio DMA ops for virtio devices in Xen guests

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Message-ID: <alpine.DEB.2.22.394.2204151305050.915916@ubuntu-linux-20-04-desktop>
Date:   Fri, 15 Apr 2022 15:02:45 -0700 (PDT)
From:   Stefano Stabellini <sstabellini@...nel.org>
To:     Oleksandr Tyshchenko <olekstysh@...il.com>
cc:     xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Julien Grall <julien@....org>
Subject: Re: [RFC PATCH 6/6] arm/xen: Assign xen-virtio DMA ops for virtio
 devices in Xen guests

On Thu, 14 Apr 2022, Oleksandr Tyshchenko wrote:
> From: Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>
> 
> Call xen_virtio_setup_dma_ops() only for Xen-aware virtio devices
> in Xen guests if restricted access to the guest memory is enabled.
> 
> Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>
> ---
>  include/xen/arm/xen-ops.h | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/include/xen/arm/xen-ops.h b/include/xen/arm/xen-ops.h
> index 621da05..28b2ad3 100644
> --- a/include/xen/arm/xen-ops.h
> +++ b/include/xen/arm/xen-ops.h
> @@ -2,12 +2,19 @@
>  #ifndef _ASM_ARM_XEN_OPS_H
>  #define _ASM_ARM_XEN_OPS_H
>  
> +#include <linux/virtio_config.h>
>  #include <xen/swiotlb-xen.h>
> +#include <xen/xen-ops.h>
>  
>  static inline void xen_setup_dma_ops(struct device *dev)
>  {
>  	if (xen_swiotlb_detect())
>  		dev->dma_ops = &xen_swiotlb_dma_ops;
> +
> +#ifdef CONFIG_XEN_VIRTIO
> +	if (arch_has_restricted_virtio_memory_access() && xen_is_virtio_device(dev))
> +		xen_virtio_setup_dma_ops(dev);
> +#endif

This makes sense overall. Considering that the swiotlb-xen case and the
virtio case are mutually exclusive, I would write it like this:

	if (arch_has_restricted_virtio_memory_access() && xen_is_virtio_device(dev))
		xen_virtio_setup_dma_ops(dev);
	else if (xen_swiotlb_detect())
		dev->dma_ops = &xen_swiotlb_dma_ops;

There is no need for #ifdef (also see other comments).