Subject: Re: [PATCH 2/2] iommu/dma: Avoid SAC address trick for PCIe devices

From: Joerg Roedel <[email-masked]>
Date: Mon, 13 Jul 2020 15:14:26 +0200

On Wed, Jul 08, 2020 at 12:32:42PM +0100, Robin Murphy wrote:
> As for the intel-iommu implementation, relegate the opportunistic
> attempt to allocate a SAC address to the domain of conventional PCI
> devices only, to avoid it increasingly causing far more performance
> issues than possible benefits on modern PCI Express systems.
> 
> Signed-off-by: Robin Murphy <[email-masked]>
> ---
>  drivers/iommu/dma-iommu.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/iommu/dma-iommu.c b/drivers/iommu/dma-iommu.c
> index 4959f5df21bd..0ff124f16ad4 100644
> --- a/drivers/iommu/dma-iommu.c
> +++ b/drivers/iommu/dma-iommu.c
> @@ -426,7 +426,8 @@ static dma_addr_t iommu_dma_alloc_iova(struct iommu_domain *domain,
>  		dma_limit = min(dma_limit, (u64)domain->geometry.aperture_end);
>  
>  	/* Try to get PCI devices a SAC address */
> -	if (dma_limit > DMA_BIT_MASK(32) && dev_is_pci(dev))
> +	if (dma_limit > DMA_BIT_MASK(32) &&
> +	    dev_is_pci(dev) && !pci_is_pcie(to_pci_dev(dev)))
>  		iova = alloc_iova_fast(iovad, iova_len,
>  				       DMA_BIT_MASK(32) >> shift, false);
>  

Unfortunatly this patch causes XHCI initialization failures on my AMD
Ryzen system. I will remove both from the IOMMU tree for now.

I guess the XHCI chip in my system does not support full 64bit dma
addresses and needs a quirk or something like that. But until this is
resolved its better to not change the IOVA allocation behavior.

Regards,

	Joerg

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