linux-kernel - Re: [PATCH 2/4] x86: always initialize xen-swiotlb when xen-pcifront is enabling

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Date:   Thu, 18 May 2023 20:18:39 +0200
From:   Marek Marczykowski-Górecki 
        <marmarek@...isiblethingslab.com>
To:     Christoph Hellwig <hch@....de>
Cc:     Juergen Gross <jgross@...e.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, Ben Skeggs <bskeggs@...hat.com>,
        Karol Herbst <kherbst@...hat.com>,
        Lyude Paul <lyude@...hat.com>, xen-devel@...ts.xenproject.org,
        iommu@...ts.linux.dev, linux-kernel@...r.kernel.org,
        nouveau@...ts.freedesktop.org
Subject: Re: [PATCH 2/4] x86: always initialize xen-swiotlb when xen-pcifront
 is enabling

On Thu, May 18, 2023 at 03:42:51PM +0200, Christoph Hellwig wrote:
> Remove the dangerous late initialization of xen-swiotlb in
> pci_xen_swiotlb_init_late and instead just always initialize
> xen-swiotlb in the boot code if CONFIG_XEN_PCIDEV_FRONTEND is enabled.
> 
> Signed-off-by: Christoph Hellwig <hch@....de>

Doesn't it mean all the PV guests will basically waste 64MB of RAM
by default each if they don't really have PCI devices?

> ---
>  arch/x86/include/asm/xen/swiotlb-xen.h |  6 ------
>  arch/x86/kernel/pci-dma.c              | 25 +++----------------------
>  drivers/pci/xen-pcifront.c             |  6 ------
>  3 files changed, 3 insertions(+), 34 deletions(-)
> 
> diff --git a/arch/x86/include/asm/xen/swiotlb-xen.h b/arch/x86/include/asm/xen/swiotlb-xen.h
> index 77a2d19cc9909e..abde0f44df57dc 100644
> --- a/arch/x86/include/asm/xen/swiotlb-xen.h
> +++ b/arch/x86/include/asm/xen/swiotlb-xen.h
> @@ -2,12 +2,6 @@
>  #ifndef _ASM_X86_SWIOTLB_XEN_H
>  #define _ASM_X86_SWIOTLB_XEN_H
>  
> -#ifdef CONFIG_SWIOTLB_XEN
> -extern int pci_xen_swiotlb_init_late(void);
> -#else
> -static inline int pci_xen_swiotlb_init_late(void) { return -ENXIO; }
> -#endif
> -
>  int xen_swiotlb_fixup(void *buf, unsigned long nslabs);
>  int xen_create_contiguous_region(phys_addr_t pstart, unsigned int order,
>  				unsigned int address_bits,
> diff --git a/arch/x86/kernel/pci-dma.c b/arch/x86/kernel/pci-dma.c
> index f887b08ac5ffe4..c4a7ead9eb674e 100644
> --- a/arch/x86/kernel/pci-dma.c
> +++ b/arch/x86/kernel/pci-dma.c
> @@ -81,27 +81,6 @@ static void __init pci_xen_swiotlb_init(void)
>  	if (IS_ENABLED(CONFIG_PCI))
>  		pci_request_acs();
>  }
> -
> -int pci_xen_swiotlb_init_late(void)
> -{
> -	if (dma_ops == &xen_swiotlb_dma_ops)
> -		return 0;
> -
> -	/* we can work with the default swiotlb */
> -	if (!io_tlb_default_mem.nslabs) {
> -		int rc = swiotlb_init_late(swiotlb_size_or_default(),
> -					   GFP_KERNEL, xen_swiotlb_fixup);
> -		if (rc < 0)
> -			return rc;
> -	}
> -
> -	/* XXX: this switches the dma ops under live devices! */
> -	dma_ops = &xen_swiotlb_dma_ops;
> -	if (IS_ENABLED(CONFIG_PCI))
> -		pci_request_acs();
> -	return 0;
> -}
> -EXPORT_SYMBOL_GPL(pci_xen_swiotlb_init_late);
>  #else
>  static inline void __init pci_xen_swiotlb_init(void)
>  {
> @@ -111,7 +90,9 @@ static inline void __init pci_xen_swiotlb_init(void)
>  void __init pci_iommu_alloc(void)
>  {
>  	if (xen_pv_domain()) {
> -		if (xen_initial_domain() || x86_swiotlb_enable)
> +		if (xen_initial_domain() ||
> +		    IS_ENABLED(CONFIG_XEN_PCIDEV_FRONTEND) ||
> +		    x86_swiotlb_enable)
>  			pci_xen_swiotlb_init();
>  		return;
>  	}
> diff --git a/drivers/pci/xen-pcifront.c b/drivers/pci/xen-pcifront.c
> index 83c0ab50676dff..11636634ae512f 100644
> --- a/drivers/pci/xen-pcifront.c
> +++ b/drivers/pci/xen-pcifront.c
> @@ -22,7 +22,6 @@
>  #include <linux/bitops.h>
>  #include <linux/time.h>
>  #include <linux/ktime.h>
> -#include <linux/swiotlb.h>
>  #include <xen/platform_pci.h>
>  
>  #include <asm/xen/swiotlb-xen.h>
> @@ -669,11 +668,6 @@ static int pcifront_connect_and_init_dma(struct pcifront_device *pdev)
>  
>  	spin_unlock(&pcifront_dev_lock);
>  
> -	if (!err && !is_swiotlb_active(&pdev->xdev->dev)) {
> -		err = pci_xen_swiotlb_init_late();
> -		if (err)
> -			dev_err(&pdev->xdev->dev, "Could not setup SWIOTLB!\n");
> -	}
>  	return err;
>  }
>  
> -- 
> 2.39.2
> 
> 

-- 
Best Regards,
Marek Marczykowski-Górecki
Invisible Things Lab

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