Re: Enable more than 255 VCPU support without irq remapping function in the guest

[Jan Kiszka <jan.kiszka@siemens.com>]

On 2016-05-04 03:46, Yang Zhang wrote:
> On 2016/5/3 12:55, Jan Kiszka wrote:
>> On 2016-05-03 04:03, Nadav Amit wrote:
>>>
>>> Anyhow, the VM can use IOMMU passthrough mode to avoid most IOMMU
>>> overhead.
>>> Regardless, a recent patch-set should improve DMAR performance
>>> considerably [1].
>>
>> The bottleneck with emulated DMAR is rather in QEMU. But DMAR can be
>> almost as cheap as IR once we get it running for VFIO and vhost: both
>> need proper caching because they do not work with QEMU in the loop for
>> each and every DMA transfer. Still no need to deviate from physical
>> hardware.
> 
> Sorry, i don't know detail about how VFIO and vhost work with IR. But it
> seems hard to do proper caching since DMA allocations are very
> frequently in Linux unless we move the whole iommu emulation to kernel.

There is technically no reason for Linux to reprogram the DMAR units
unless it changes partitioning (or really wants to enforce strict DMA
containment for each device). You can surely tune this to no updates at
all for the guest Linux under normal operations.

Jan

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