linux-kernel - Re: [PATCH 3/3] iommu: Disable passthrough mode when SME is active

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Date:   Fri, 9 Aug 2019 16:50:48 +0000
From:   "Lendacky, Thomas" <Thomas.Lendacky@....com>
To:     Joerg Roedel <joro@...tes.org>
CC:     "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Suthikulpanit, Suravee" <Suravee.Suthikulpanit@....com>,
        "bp@...en8.de" <bp@...en8.de>, Joerg Roedel <jroedel@...e.de>
Subject: Re: [PATCH 3/3] iommu: Disable passthrough mode when SME is active

On 8/9/19 10:22 AM, Joerg Roedel wrote:
> From: Joerg Roedel <jroedel@...e.de>
> 
> Using Passthrough mode when SME is active causes certain
> devices to use the SWIOTLB bounce buffer. The bounce buffer
> code has an upper limit of 256kb for the size of DMA
> allocations, which is too small for certain devices and
> causes them to fail.
> 
> With this patch we enable IOMMU by default when SME is
> active in the system, making the default configuration work
> for more systems than it does now.
> 
> Users that don't want IOMMUs to be enabled still can disable
> them with kernel parameters.
> 
> Signed-off-by: Joerg Roedel <jroedel@...e.de>
> ---
>   drivers/iommu/iommu.c | 6 ++++++
>   1 file changed, 6 insertions(+)
> 
> diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
> index 62cae6db0970..fbe1aa51bce9 100644
> --- a/drivers/iommu/iommu.c
> +++ b/drivers/iommu/iommu.c
> @@ -104,6 +104,12 @@ static int __init iommu_subsys_init(void)
>   	else
>   		iommu_def_domain_type = IOMMU_DOMAIN_DMA;
>   
> +	if ((iommu_def_domain_type == IOMMU_DOMAIN_IDENTITY) &&
> +	    sme_active()) {
> +		pr_info("SME detected - Disabling default IOMMU passthrough\n");
> +		iommu_def_domain_type = IOMMU_DOMAIN_DMA;

Should this also clear the iommu_pass_through variable (the one set by the
iommu kernel parameter in arch/x86/kernel/pci-dma.c)?

I guess this is more applicable to the original patchset that created the
CONFIG_IOMMU_DEFAULT_PASSTHROUGH option, but should the default
passthrough support be modified so that you don't have to specify multiple
kernel parameters to change it?

Right now, if CONFIG_IOMMU_DEFAULT_PASSTHROUGH is set to yes, you can't
just specify iommu=nopt to enable the IOMMU. You have to also specify
iommu.passthrough=0. Do we want to fix that so that just specifying
iommu=nopt or iommu.passthrough=0 does what is needed?

Thanks,
Tom

> +	}
> +
>   	pr_info("Default domain type: %s\n",
>   		iommu_domain_type_str(iommu_def_domain_type));
>   
>