linux-kernel - Re: [PATCH 2/4] iommu: Use group ownership to avoid driver attachment

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Message-ID: <4ac228af-fb74-2d89-b8d7-462186170e12@linux.intel.com>
Date:   Wed, 15 Feb 2023 15:28:23 +0800
From:   Baolu Lu <baolu.lu@...ux.intel.com>
To:     "Tian, Kevin" <kevin.tian@...el.com>,
        Jason Gunthorpe <jgg@...dia.com>
Cc:     baolu.lu@...ux.intel.com,
        "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
        Joerg Roedel <joro@...tes.org>,
        Christoph Hellwig <hch@...radead.org>,
        Will Deacon <will@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/4] iommu: Use group ownership to avoid driver attachment

On 2023/2/15 14:56, Tian, Kevin wrote:
>> From: Baolu Lu<baolu.lu@...ux.intel.com>
>> Sent: Wednesday, February 15, 2023 1:51 PM
>>
>> On 2/13/23 10:19 PM, Jason Gunthorpe wrote:
>>> On Mon, Feb 13, 2023 at 03:49:39PM +0800, Lu Baolu wrote:
>>>> @@ -2992,6 +2987,14 @@ static ssize_t iommu_group_store_type(struct
>> iommu_group *group,
>>>>    	else
>>>>    		return -EINVAL;
>>>>
>>>> +	if (req_type != IOMMU_DOMAIN_DMA_FQ ||
>>>> +	    group->default_domain->type != IOMMU_DOMAIN_DMA) {
>>>> +		ret = iommu_group_claim_dma_owner(group, (void *)buf);
>>>> +		if (ret)
>>>> +			return ret;
>>>> +		group_owner_claimed = true;
>>>> +	}
>>> I don't get it, this should be done unconditionally. If we couldn't
>>> take ownership then we simply can't progress.
>> The existing code allows the user to switch the default domain from
>> strict to lazy invalidation mode. The default domain is not changed,
>> hence it should be seamless and transparent to the device driver.
> Is there real usage relying on this transition for a bound device?
> 
> In concept strict->lazy transition implies relaxed DMA security. It's hard
> to think of a motivation of doing so while the device might be doing
> in-fly DMAs.
> 
> Presumably such perf/security tradeoff should be planned way before
> binding device/driver together.
> 
> btw if strict->lazy is allowed why lazy->strict is prohibited?
> 

We all know, strict vs. lazy is a tradeoff between performance and
security.

strict -> lazy: driver works in secure mode. This transition trades off
security for better performance.

lazy->strict: The driver is already working in non-safety mode. This
transition only results in worse performance. It makes no sense. If user
want to put the driver in a secure mode, they need to unbind the driver,
reset the device and do the lazy->strict transition.

Robin might have better insights.

Best regards,
baolu