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

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Message-ID: <f4559966-c99e-47d2-a5ab-d248c7ed8e52@arm.com>
Date:   Wed, 15 Feb 2023 11:09:22 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Baolu Lu <baolu.lu@...ux.intel.com>,
        "Tian, Kevin" <kevin.tian@...el.com>,
        Jason Gunthorpe <jgg@...dia.com>
Cc:     "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
        Joerg Roedel <joro@...tes.org>,
        Christoph Hellwig <hch@...radead.org>,
        Will Deacon <will@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/4] iommu: Use group ownership to avoid driver attachment

On 2023-02-15 07:28, Baolu Lu wrote:
> 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.

Yes, this was added for a definite use-case in ChromeOS, where 
strict->lazy needs to support being done "live" since the device in 
question is the storage controller for the already-mounted root 
filesystem. Your reasoning seems to match what I summarised in the 
original commit message :)

Thanks,
Robin.