linux-kernel - Re: [PATCH 4/4] powerpc/pseries/iommu: Remove default DMA window before creating DDW

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Date:   Mon, 22 Jun 2020 20:02:51 +1000
From:   Alexey Kardashevskiy <aik@...abs.ru>
To:     Leonardo Bras <leobras.c@...il.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Thiago Jung Bauermann <bauerman@...ux.ibm.com>,
        Ram Pai <linuxram@...ibm.com>
Cc:     linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/4] powerpc/pseries/iommu: Remove default DMA window
 before creating DDW



On 19/06/2020 15:06, Leonardo Bras wrote:
> On LoPAR "DMA Window Manipulation Calls", it's recommended to remove the
> default DMA window for the device, before attempting to configure a DDW,
> in order to make the maximum resources available for the next DDW to be
> created.
> 
> This is a requirement for some devices to use DDW, given they only
> allow one DMA window.
> 
> If setting up a new DDW fails anywhere after the removal of this
> default DMA window, restore it using reset_dma_window.

Nah... If we do it like this, then under pHyp we lose 32bit DMA for good
as pHyp can only create a single window and it has to map at
0x800.0000.0000.0000. They probably do not care though.

Under KVM, this will fail as VFIO allows creating  2 windows and it
starts from 0 but the existing iommu_bypass_supported_pSeriesLP() treats
the window address == 0 as a failure. And we want to keep both DMA
windows for PCI adapters with both 64bit and 32bit PCI functions (I
heard AMD GPU video + audio are like this) or someone could hotplug
32bit DMA device on a vphb with already present 64bit DMA window so we
do not remove the default window.

The last discussed thing I remember was that there was supposed to be a
new bit in "ibm,architecture-vec-5" (forgot the details), we could use
that to decide whether to keep the default window or not, like this.

> 
> Signed-off-by: Leonardo Bras <leobras.c@...il.com>
> ---
>  arch/powerpc/platforms/pseries/iommu.c | 20 +++++++++++++++++---
>  1 file changed, 17 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/powerpc/platforms/pseries/iommu.c b/arch/powerpc/platforms/pseries/iommu.c
> index de633f6ae093..68d1ea957ac7 100644
> --- a/arch/powerpc/platforms/pseries/iommu.c
> +++ b/arch/powerpc/platforms/pseries/iommu.c
> @@ -1074,8 +1074,9 @@ static u64 enable_ddw(struct pci_dev *dev, struct device_node *pdn)
>  	u64 dma_addr, max_addr;
>  	struct device_node *dn;
>  	u32 ddw_avail[3];
> +
>  	struct direct_window *window;
> -	struct property *win64;
> +	struct property *win64, *dfl_win;

Make it "default_win" or "def_win", "dfl" hurts to read :)

>  	struct dynamic_dma_window_prop *ddwprop;
>  	struct failed_ddw_pdn *fpdn;
>  
> @@ -1110,8 +1111,19 @@ static u64 enable_ddw(struct pci_dev *dev, struct device_node *pdn)
>  	if (ret)
>  		goto out_failed;
>  
> -       /*
> -	 * Query if there is a second window of size to map the
> +	/*
> +	 * First step of setting up DDW is removing the default DMA window,
> +	 * if it's present. It will make all the resources available to the
> +	 * new DDW window.
> +	 * If anything fails after this, we need to restore it.
> +	 */
> +
> +	dfl_win = of_find_property(pdn, "ibm,dma-window", NULL);
> +	if (dfl_win)
> +		remove_dma_window(pdn, ddw_avail, dfl_win);

Before doing so, you want to make sure that the "reset" is actually
supported. Thanks,


> +
> +	/*
> +	 * Query if there is a window of size to map the
>  	 * whole partition.  Query returns number of windows, largest
>  	 * block assigned to PE (partition endpoint), and two bitmasks
>  	 * of page sizes: supported and supported for migrate-dma.
> @@ -1219,6 +1231,8 @@ static u64 enable_ddw(struct pci_dev *dev, struct device_node *pdn)
>  	kfree(win64);
>  
>  out_failed:
> +	if (dfl_win)
> +		reset_dma_window(dev, pdn);
>  
>  	fpdn = kzalloc(sizeof(*fpdn), GFP_KERNEL);
>  	if (!fpdn)
> 

-- 
Alexey