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

From: Alexey Kardashevskiy
Date: Mon Jun 22 2020 - 06:03:03 EST




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@xxxxxxxxx>
> ---
> 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