Re: question: xen/qemu - mmio mapping issues for device pass-through

["Xuquan (Quan Xu)" <xuquan8@huawei.com>]

On March 16, 2017 10:06 PM, Jan Beulich wrote:
>>>> On 16.03.17 at 14:55, <xuquan8@huawei.com> wrote:
>> I try to pass-through a device with 8G large bar, such as nvidia
>> M60(note1, pci-e info as below). It takes about '__15 sconds__' to
>> update 8G large bar in QEMU::xen_pt_region_update()..
>> Specifically, it is xc_domain_memory_mapping() in xen_pt_region_update().
>>
>> Digged into xc_domain_memory_mapping(), I find it mainly call
>> "do_domctl
>> （…case XEN_DOMCTL_memory_mapping…）"
>> to mapping mmio region.. of cause, I find out that this mapping could
>> take a while in the code comment below ' case
>XEN_DOMCTL_memory_mapping '.
>>
>> my questions:
>> 1. could we make this mapping mmio region quicker?
>

Thanks for your quick reply.

>Yes, e.g. by using large (2M or 1G) pages. This has been on my todo list for
>quite a while...
>
>> 2. if could not, does it limit by hardware performance?
>
>I'm afraid I don't understand the question. If you mean "Is it limited by hw
>performance", then no, see above. If you mean "Does it limit hw performance",
>then again no, I don't think so (other than the effect of having more IOMMU
>translation levels than really necessary for such large a region).
>

Sorry, my question is  "Is it limited by hw performance"... 

I am much confused. why does this mmio mapping take a while?
I guessed it takes a lot of time to set up p2m / iommu entry. That's why I ask "Is it limited by hw performance".

Quan
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