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

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

On 2016-05-03 04:03, Nadav Amit wrote:
> Yang Zhang <yang.zhang.wz@gmail.com> wrote:
> 
>> I think it is not only interrupt. There must have the DMAR emulation and
>> the cost for DMA is heavy in VM(DMA operations are very frequently). I
>> cannot remember whether there are strong dependency in hardware between
>> DMAR and IR(I know IR is relying on QI). Even hardware dependency is ok,
>> is it ok for OS running in hardware with IR but without DMAR?
> 
> Do you know a way for the IOMMU to report that DMAR is disabled, while IR
> is enabled?

The hardware cannot decide about disabling this, but the guest can, of
course. In fact, you can even configure Linux to have DMAR off by
default until you pass "intel_iommu=on" on the command line (I think
distros still do this - at least they used to). No idea about other
OSes, though.

> 
> 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.

Jan

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