Re: Opinions AMD64 vs EM64T vs Itanium II

> 
> I assume that the following comment from the RHEL3 U2 update is still valid or
> has something changed with woodcrest?
> 
> “Software IOTLB — Intel EM64T does not support an IOMMU in hardware while AMD64
> processors do. This means that physical addresses above 4GB (32 bits) cannot
> reliably be the source or destination of DMA operations. Therefore, the Red Hat
> Enterprise Linux 3 Update 2 kernel "bounces" all DMA operations to or from
> physical addresses above 4GB to buffers that the kernel pre-allocated below 4GB
> at boot time. This is likely to result in lower performance for IO-intensive
> workloads for Intel EM64T as compared to AMD64 processors.”

this is only true if you have PCI devices that are 32 bit only. While
this holds for most USB, this isn't generally true for any modern (as in
"last 10 years") scsi or AHCI SATA controller, nor is it true for
network controllers. All these high speed controllers are 64 bit capable
and don't use (or need) an iommu, not even on opteron.


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