Re: [RFC] /dev/ioasid uAPI proposal

Jason Wang
2021-06-03T02:53:05Z

在 2021/6/3 上午4:37, Alex Williamson 写道:
> On Wed, 2 Jun 2021 16:54:04 -0300
> Jason Gunthorpe <jgg@nvidia.com> wrote:
>
>> On Wed, Jun 02, 2021 at 01:00:53PM -0600, Alex Williamson wrote:
>>> Right, the device can generate the no-snoop transactions, but it's the
>>> IOMMU that essentially determines whether those transactions are
>>> actually still cache coherent, AIUI.
>> Wow, this is really confusing stuff in the code.
>>
>> At the PCI level there is a TLP bit called no-snoop that is platform
>> specific. The general intention is to allow devices to selectively
>> bypass the CPU caching for DMAs. GPUs like to use this feature for
>> performance.
> Yes
>
>> I assume there is some exciting security issues here. Looks like
>> allowing cache bypass does something bad inside VMs? Looks like
>> allowing the VM to use the cache clear instruction that is mandatory
>> with cache bypass DMA causes some QOS issues? OK.
> IIRC, largely a DoS issue if userspace gets to choose when to emulate
> wbinvd rather than it being demanded for correct operation.
>
>> So how does it work?
>>
>> What I see in the intel/iommu.c is that some domains support "snoop
>> control" or not, based on some HW flag. This indicates if the
>> DMA_PTE_SNP bit is supported on a page by page basis or not.
>>
>> Since x86 always leans toward "DMA cache coherent" I'm reading some
>> tea leaves here:
>>
>> IOMMU_CAP_CACHE_COHERENCY, /* IOMMU can enforce cache coherent DMA
>> transactions */
>>
>> And guessing that IOMMUs that implement DMA_PTE_SNP will ignore the
>> snoop bit in TLPs for IOVA's that have DMA_PTE_SNP set?
> That's my understanding as well.
>
>> Further, I guess IOMMUs that don't support PTE_SNP, or have
>> DMA_PTE_SNP clear will always honour the snoop bit. (backwards compat
>> and all)
> Yes.
>
>> So, IOMMU_CAP_CACHE_COHERENCY does not mean the IOMMU is DMA
>> incoherent with the CPU caches, it just means that that snoop bit in
>> the TLP cannot be enforced. ie the device *could* do no-shoop DMA
>> if it wants. Devices that never do no-snoop remain DMA coherent on
>> x86, as they always have been.
> Yes, IOMMU_CAP_CACHE_COHERENCY=false means we cannot force the device
> DMA to be coherent via the IOMMU.
>
>> IOMMU_CACHE does not mean the IOMMU is DMA cache coherent, it means
>> the PCI device is blocked from using no-snoop in its TLPs.
>>
>> I wonder if ARM implemented this consistently? I see VDPA is
>> confused..

Basically, we don't want to bother with pseudo KVM device like what VFIO
did. So for simplicity, we rules out the IOMMU that can't enforce
coherency in vhost-vDPA if the parent purely depends on the platform IOMMU:

if (!iommu_capable(bus, IOMMU_CAP_CACHE_COHERENCY))
return -ENOTSUPP;

For the parents that use its own translations logic, an implicit
assumption is that the hardware will always perform cache coherent DMA.

Thanks