[PATCH 1/2] VT-d: Support multiple device assignment for KVM

[Yu, Fenghua]

Avi Kivity wrote:
>> Han, Weidong wrote:
>>> In order to support multiple device assignment for KVM, this patch
>>> does following main changes: 
>>>    - extend dmar_domain to own multiple devices from different
>>> iommus, use a bitmap of iommus to replace iommu pointer in
>>> dmar_domain.  
>>>    - add a flag DOMAIN_FLAG_VIRTUAL_MACHINE to represent KVM VT-d
>>> usage. Many functions (e.g. intel_map_single() and
>>> intel_unmap_single()) won't be used by KVM VT-d. Let them return
>>> directly when this flag is set.   
>>> 
>> 
>> 
>> This seems brittle.  An API that has some functions shorted out
>> depending on some flag is hard to understand and use.

>This flag just helps identify kvm VT-d usage, and let kvm VT-d APIs reuse >native VT-d code, it needn't duplicate code for kvm VT-d APIs, and it won't >impact existed native VT-d code. 

>> 
>> We should either implement the functions, or split the API into a
>> basic version that talks only to one device, and an expanded versions
>> that talks to multiple devices, and is implemented by the using the
>> lower level API.  This may require more changes due to the need to
>> share io pagetables.

>The expanded versions that supports multiple devices will need to change >dmar_domain, this will cause lots of changes, almost duplicate the main >functions.

I would agree with Avi. It's wired to merge the KVM api and native api together while they are logically separate. If we have two sets of interfaces for KVM and native iommu each, code is clean and robust and easy to extend in the future. If you well organize the code, we should have both good interface and shared code. While it makes sense to share code at beginning of KVM like this, it will get harder to maintain and evolve both KVM and native later, e.g. changing one part may break the other.

Some comments on the code in this patch:

1. You don't need to check DOMAIN_FLAG_VIRTUAL_MACHINE in the DMA functions. When the DMA functions are called, the domain should not have the DOMAIN_FLAG_VIRTUAL_MACHINE flag. Otherwise, the domain allocation code is wrong.

2. You need to initialize domain->flags=0 for DMA domain. It's random number after the domain is allocated by kmem_cache_alloc.

Thanks.

-Fenghua