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IOMMU中断remapping的作用  

2015-09-02 13:59:57|  分类: VT-related |  标签: |举报 |字号 订阅

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问了linux community,IOMMU interrupt remapping的,两个作用:

---------- Forwarded message ----------
From: Alex Williamson <alex.williamson@redhat.com>
Date: Wed, Nov 2, 2011 at 11:31 PM
Subject: Re: What's the usage model (purpose) of interrupt remapping in IOMMU?
To: Kai Huang <mail.kai.huang@gmail.com>
Cc: kvm@vger.kernel.org, linux-pci@ger.kernel.org


On Wed, 2011-11-02 at 13:26 +0800, Kai Huang wrote:
> Hi,
>
> In case of direct io, without the interrupt remapping in IOMMU (intel
> VT-d or AMD IOMMU), hypervisor needs to inject interrupt for guest
> when the guest is scheduled to specific CPU. At the beginning I
> thought with IOMMU's interrupt remapping, the hardware can directly
> forward the interrupt to guest without trapping into hypervisor when
> the interrupt happens, but after reading the Intel VT-d's manual, I
> found the interrupt mapping feature just add another mapping which
> allows software to control (mainly) the destination and vector, and we
> still need hypervisor to inject the interrupt when the guest is
> scheduled as only after the guest is scheduled, the target CPU can be
> known. If my understanding is correct, seems the interrupt remapping
> does not bring any performance improvement. So what's the benefit of
> IOMMU's interrupt remapping? Can someone explain the usage model of
> interrupt remapping in IOMMU?

Interrupt remapping provides isolation and compatibility, not
performance.  The hypervisor being able to direct interrupts to a target
CPU also allows it the ability to filter interrupts and prevent the
device from signaling spurious or malicious interrupts.  This is
particularly important with message signaled interrupts since any device
capable of DMA is able to inject random MSIs into the host.  The
compatibility side is a feature of Intel platforms supporting x2apic.
The interrupt remapper provides a translation layer to allow xapic aware
hardware, such as ioapics, to function when the processors are switched
to x2apic mode.  Thanks,

Alex
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