linux-kernel - RE: [RFC v1 2/2] powerpc/pseries/iommu: Use dma_iommu

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Date:   Thu, 7 Nov 2019 21:49:22 -0800
From:   Ram Pai <linuxram@...ibm.com>
To:     Michael Ellerman <mpe@...erman.id.au>
Cc:     linuxppc-dev@...ts.ozlabs.org, benh@...nel.crashing.org,
        david@...son.dropbear.id.au, paulus@...abs.org,
        mdroth@...ux.vnet.ibm.com, hch@....de, andmike@...ibm.com,
        sukadev@...ux.vnet.ibm.com, mst@...hat.com, ram.n.pai@...il.com,
        aik@...abs.ru, cai@....pw, tglx@...utronix.de,
        bauerman@...ux.ibm.com, linux-kernel@...r.kernel.org
Subject: RE: [RFC v1 2/2] powerpc/pseries/iommu: Use dma_iommu_ops for Secure VMs
 aswell.

On Thu, Nov 07, 2019 at 09:26:28PM +1100, Michael Ellerman wrote:
> Ram Pai <linuxram@...ibm.com> writes:
> > This enables IOMMU support for pseries Secure VMs.
> 
> Can you give us some more explanation please?

Yes. Will do. 

The simple explanation is -- it was a mistake. We should 
not have disabled IOMMU ops for secure guests. Though it enabled
us to use virtio devices, with the help of some additional patches to
the virtio subsystem; in hindsight, we should not have disabled IOMMU
ops for secure VMs  :-(. 

RP



> 
> This is basically a revert of commit:
>   edea902c1c1e ("powerpc/pseries/iommu: Don't use dma_iommu_ops on secure guests")
> 
> But neglects to remove the now unnecessary include of svm.h.
> 
> > diff --git a/arch/powerpc/platforms/pseries/iommu.c b/arch/powerpc/platforms/pseries/iommu.c
> > index 07f0847..189717b 100644
> > --- a/arch/powerpc/platforms/pseries/iommu.c
> > +++ b/arch/powerpc/platforms/pseries/iommu.c
> > @@ -1333,15 +1333,7 @@ void iommu_init_early_pSeries(void)
> >  	of_reconfig_notifier_register(&iommu_reconfig_nb);
> >  	register_memory_notifier(&iommu_mem_nb);
> >  
> > -	/*
> > -	 * Secure guest memory is inacessible to devices so regular DMA isn't
> > -	 * possible.
> > -	 *
> > -	 * In that case keep devices' dma_map_ops as NULL so that the generic
> > -	 * DMA code path will use SWIOTLB to bounce buffers for DMA.
> 
> Please explain what has changed to make this no longer necessary.
> 
> cheers
> 
> > -	 */
> > -	if (!is_secure_guest())
> > -		set_pci_dma_ops(&dma_iommu_ops);
> > +	set_pci_dma_ops(&dma_iommu_ops);
> >  }
> >  
> >  static int __init disable_multitce(char *str)
> > -- 
> > 1.8.3.1

-- 
Ram Pai