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

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Message-ID: <4bf1d32da3d13a44e3c2e4b04f369fe52c24a023.camel@gmail.com>
Date:   Mon, 22 Jun 2020 15:59:28 -0300
From:   Leonardo Bras <leobras.c@...il.com>
To:     Alexey Kardashevskiy <aik@...abs.ru>,
        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

Hello Alexey, thanks for the feedback!

On Mon, 2020-06-22 at 20:02 +1000, Alexey Kardashevskiy wrote:
> 
> 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.

Well, then I would suggest doing something like this:
	query_ddw(...);
  	if (query.windows_available == 0){
		remove_dma_window(...,default_win);
		query_ddw(...);
	}

This would make sure to cover cases of windows available == 1
and windows available > 1; 

> 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.

I checked on the latest LoPAR draft (soon to be published), for the
ibm,architecture-vec 'option array 5' and this entry was the only
recently added one that is related to this patchset:

Byte 8 - Bit 0:
SRIOV Virtual Functions Support Dynamic DMA Windows (DDW):
0: SRIOV Virtual Functions do not support DDW
1: SRIOV Virtual Functions do support DDW

Isn't this equivalent to having a "ibm,ddw-applicable" property?


> 
> > 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 :)

Sure, no problem :)

> 
> >  	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,

Good catch, I will improve that.

> 
> 
> > +
> > +	/*
> > +	 * 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)
> > 

Best regards,
Leonardo