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

[Leonardo Bras <leobras.c@gmail.com>]

On Tue, 2020-06-23 at 11:11 +1000, Alexey Kardashevskiy wrote:
> 
> On 23/06/2020 04:59, Leonardo Bras wrote:
> > 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; 
> 
> Is "1" what pHyp returns on query? And was it always like that? Then it
> is probably ok. I just never really explored the idea of removing the
> default window as we did not have to.

I am not sure if this is true in general, but in this device (SR-IOV
VF) I am testing it will return 0 windows if the default DMA window is
not deleted, and 1 after it's deleted.

> 
> 
> > > 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?
> 
> I am not sure, is there anything else to this new bit?

I copied everything from the LoPAR, and IIRC the ACR for this change
only described this change in the document.


>  I'd think if the
> client supports it, then pHyp will create one 64bit window per a PE and
> DDW API won't be needed. Thanks,

That would make sense, and be great.
I will dig some more.

Thank you!

> > > > Signed-off-by: Leonardo Bras <leobras.c@gmail.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
> >