DMA understanding

[Abhinav Srivastava <abhinavs_iitkgp@yahoo.co.in>]

Hi there,

I am trying to understand how an HVM guest domain performs its DMA operations, and how this DMA operations are intercepted by the Xen. I wanted to understand both the code path with and without Vt-d support (for intel processors). On looking inside the Xen code, I found that iommu code is inside the vmx/vtd/ directory only. By seeing the code, my understanding is that when Vt-d is enabled, iommu.c and dmar.c inside the vtd directory is the place to look for DMA operations. However, I do not understand which code path inside the hypervisor is getting used in case of Vt-d is disabled?  How does Xen intercept guest DMA operations in this case? I am using Xen 3.3 version for my project (I admit that it is very old version).

Any help would be appreciated.

Thanks,
Abhinav