linux-kernel - Re: [PATCH 3/6] iommu/vt-d: Don't force 32bit devices to uses DMA domain

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Message-ID: <b86e2bce-9907-05d0-b937-4b120797ba06@linux.intel.com>
Date:   Sun, 8 Mar 2020 10:15:48 +0800
From:   Lu Baolu <baolu.lu@...ux.intel.com>
To:     Christoph Hellwig <hch@....de>
Cc:     baolu.lu@...ux.intel.com, Joerg Roedel <joro@...tes.org>,
        ashok.raj@...el.com, jacob.jun.pan@...ux.intel.com,
        kevin.tian@...el.com, iommu@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, Daniel Drake <drake@...lessm.com>,
        Derrick Jonathan <jonathan.derrick@...el.com>,
        Jerry Snitselaar <jsnitsel@...hat.com>,
        Robin Murphy <robin.murphy@....com>
Subject: Re: [PATCH 3/6] iommu/vt-d: Don't force 32bit devices to uses DMA
 domain

Hi Christoph,

On 2020/3/7 22:21, Christoph Hellwig wrote:
> On Sat, Mar 07, 2020 at 02:20:11PM +0800, Lu Baolu wrote:
>> Currently, if a 32bit device initially uses an identity domain,
>> Intel IOMMU driver will convert it forcibly to a DMA one if its
>> address capability is not enough for the whole system memory.
>> The motivation was to overcome the overhead caused by possible
>> bounced buffer.
>>
>> Unfortunately, this improvement has led to many problems. For
>> example, some 32bit devices are required to use an identity
>> domain, forcing them to use DMA domain will cause the device
>> not to work anymore. On the other hand, the VMD sub-devices
>> share a domain but each sub-device might have different address
>> capability. Forcing a VMD sub-device to use DMA domain blindly
>> will impact the operation of other sub-devices without any
>> notification. Further more, PCI aliased devices (PCI bridge
>> and all devices beneath it, VMD devices and various devices
>> quirked with pci_add_dma_alias()) must use the same domain.
>> Forcing one device to switch to DMA domain during runtime
>> will cause in-fligh DMAs for other devices to abort or target
>> to other memory which might cause undefind system behavior.
> 
> I still don't like the idea to enforce either a strict dynamic
> IOMMU mapping or an identify mapping mode.
> 
> Can we add a new AUTO domain which will allow using the identity
> mapping when available?  That somewhat matches the existing x86
> default, and also what powerpc does.

Sai is proposing a series to change the default domain through sysfs
during runtime.

https://lore.kernel.org/linux-iommu/FFF73D592F13FD46B8700F0A279B802F4FBF7E4B@ORSMSX114.amr.corp.intel.com/T/#mb919da5567da7692ee7058a00a137145adf950a1

It has evolved into v2. Not sure whether it's what you want.

> I have a series to lift
> that bypass mode into the core dma-mapping code that I need
> to repost, which I think would be suitable for intel-iommu as well.
> 

Looking forward to your repost.

Best regards,
baolu