Re: [Xen-users] is Intel VT-d "really" necessary?

[Bruce Edge <bruce.edge@xxxxxxxxx>]

On Wed, Sep 15, 2010 at 7:38 AM, Nick Couchman <Nick.Couchman@xxxxxxxxx> wrote:
>>>> On 2010/09/15 at 03:20, Rudi Ahlers <Rudi@xxxxxxxxxxx> wrote:
>> Hi all,
>>
>> I'm just curios and would like some input from the community on this
>> one. We're busy budgeting for a couple of new servers and I thought it
>> would be good to try out the Core i7 CPU's, but see the majority of
>> them don't offer VT-d, but just VT-x. Looking at the LGA1366 range,
>> only the "Intel lga1366 i7 980XE" (from the list of what our suppliers
>> stock) have VT-d, and it costs 4x more than "Intel lga1366 i7 930" or
>> 2x more than "Intel lga1366 i7 960". From a budget perspecitve I could
>> purchase 4 more CPU's, which could translate to 40x - 80x more VM's
>> being hosted for the same capital outlay. Experience has shown that we
>> under-utilize CPU's by a great margin and memory / HDD IO is our
>> biggest bottleneck on any server.
>
> That's interesting...is it the motherboards you're seeing this lack of 
> support on, or the chips?  I'm pretty sure the i7 processors themselves 
> support VTd, you just have to have the BIOS and MB support for it.
>
>>
>> So, if VT-d really necessary?
>> We mainly host XEN virtual machine for the hosting industry, i.e. we
>> don't need / use graphics rendering inside VM's, or need DAS on the
>> VM's, etc.
>
> In an environment where you're planning on running many VMs on a single host, 
> I don't imagine VTd is going to be very helpful to you.  I use it here where 
> I work, but I use it on desktop systems where folks need access to graphics 
> cards, serial cards, etc., from Windows, and not on my server systems.  It 
> really depends on what you're doing as to whether you think you'll need it or 
> not - if you cannot think of a situation where *any* type of PCI card needs 
> to be forwarded through to an HVM domU, then you're probably okay without it. 
>  The few situations I can think of are:
> - Serial port cards, but this is really more common on desktops
> - Phone/Voicemail systems - If you're using any sort of telephony card with 
> an HVM domU you'll need direct PCI access, which requires VTd.
> - Other, vendor-specific add-in PCI cards, for applications like industrial 
> automation, etc.  But you won't see much of this in a data center.
>
> Also, keep in mind that, IIRC, PV domUs can access PCI devices without VTd, 
> so if you're going to be running PV kernels on these systems, or the PV 
> kernels you're running are the ones that need access to the PCI devices, then 
> you're probably fine without it.  I'm sure it offers some performance 
> enhancements over software-based IOMMU, but I don't know what those are.
>
> -Nick
>

Apologies in advance for hyjacking your thread, but this is an
interesting question.

For cases where PCI passthougth is the heart of the application, is it
better to use hvm/VTd or pv-ops/iommu=soft? Or is there some other
combination where you can still leverage VTd from pv-ops?

-Bruce


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