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

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Date:	Tue, 31 Jul 2012 15:46:04 +0100
From:	Stefano Stabellini <stefano.stabellini@...citrix.com>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
CC:	"fujita.tomonori@....ntt.co.jp" <fujita.tomonori@....ntt.co.jp>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Stefano Stabellini <Stefano.Stabellini@...citrix.com>,
	"JBeulich@...e.com" <JBeulich@...e.com>
Subject: Re: [PATCH 2/5] xen/swiotlb: With more than 4GB on 64-bit, disable
 the native SWIOTLB.

On Tue, 31 Jul 2012, Konrad Rzeszutek Wilk wrote:
> 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.
> 
> Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
> ---
>  arch/x86/xen/pci-swiotlb-xen.c |   12 ++++++++++++
>  1 files changed, 12 insertions(+), 0 deletions(-)
> 
> diff --git a/arch/x86/xen/pci-swiotlb-xen.c b/arch/x86/xen/pci-swiotlb-xen.c
> index b6a5340..0d5a214 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 +54,13 @@ 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 native SWIOTLB if no_iommu == 0
> +	 * (so no iommu=X command line over-writes). So disable the native
> +	 * SWIOTLB. */

Maybe rewording it would be a good idea:

/* pci_swiotlb_detect_4gb turns on native SWIOTLB if no_iommu == 0
 * (so no iommu=X command line over-writes).
 * Considering that PV guests don't normally have PCI devices it is not
 * useful to us so we set no_iommu to 1 here */


> +	if (max_pfn > MAX_DMA32_PFN)
> +		no_iommu = 1;
> +#endif
>  	return xen_swiotlb;
>  }
>  
> -- 
> 1.7.7.6
> 
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