Re: [Xen-devel] [PATCH 2/5] xen/swiotlb: With more than 4GB on 64-bit, disable the native SWIOTLB.

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Tue, 31 Jul 2012, Konrad Rzeszutek Wilk wrote:
> commit 21ef55f4ab2b6d63eb0ed86abbc959d31377853b
> Author: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> Date:   Fri Jul 27 20:16:00 2012 -0400
> 
>     xen/swiotlb: With more than 4GB on 64-bit, disable the native SWIOTLB.
>     
>     If a PV guest is booted the native SWIOTLB should not be
>     turned on. It does not help us (we don't have any PCI devices)
>     and it eats 64MB of good memory. In the case of PV guests
>     with PCI devices we need the Xen-SWIOTLB one.
>    
>     [v1: Rewrite comment per Stefano's suggestion] 
>     Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>


Acked-by: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>

> diff --git a/arch/x86/xen/pci-swiotlb-xen.c b/arch/x86/xen/pci-swiotlb-xen.c
> index b6a5340..1c17227 100644
> --- a/arch/x86/xen/pci-swiotlb-xen.c
> +++ b/arch/x86/xen/pci-swiotlb-xen.c
> @@ -8,6 +8,11 @@
>  #include <xen/xen.h>
>  #include <asm/iommu_table.h>
>  
> +#ifdef CONFIG_X86_64
> +#include <asm/iommu.h>
> +#include <asm/dma.h>
> +#endif
> +
>  int xen_swiotlb __read_mostly;
>  
>  static struct dma_map_ops xen_swiotlb_dma_ops = {
> @@ -49,6 +54,15 @@ int __init pci_xen_swiotlb_detect(void)
>        * the 'swiotlb' flag is the only one turning it on. */
>       swiotlb = 0;
>  
> +#ifdef CONFIG_X86_64
> +     /* pci_swiotlb_detect_4gb turns on native SWIOTLB if no_iommu == 0
> +      * (so no iommu=X command line over-writes).
> +      * Considering that PV guests do not want the *native SWIOTLB* but
> +      * only Xen SWIOTLB it is not useful to us so set no_iommu=1 here.
> +      */
> +     if (max_pfn > MAX_DMA32_PFN)
> +             no_iommu = 1;
> +#endif
>       return xen_swiotlb;
>  }
>  
> 

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