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Re: [Xen-devel] Difference between normal Dom0 and PVH Dom0

On Fri, Jan 23, 2015 at 03:54:07PM +0800, openlui wrote:
> Hi,  all:
>     From the article [1] and xen-colors picture from Brendan Gergg's blog 
> [2], I have some understanding and question about PVH Dom0 as follows:
>     1. Even after pvops has been support in Linux mainline kernel, current 
> XEN dom0 is still a "Full PV" domain (I will call it as "normal Dom0" in this 
> mail), am I right?

It can do both.
>     2. The normal Dom0 cannot take advantage of the Hardware visualization 
> extensions like HVM Domain, including privileged instructions and pagetables. 
> I am wondering how much performance impact it will have compared with PVH and 
> HVM Domain, and does it means that the technology such as shadow page table 
> is needed for Dom0?

It is less faster than PVH.

>     3. For the normal Dom0 running with x86_64, because of the elimination of 
> segmentation limit, each system call in Dom0 will bounce up into Xen and then 
> context-switch to the Dom0 kernel, and will cause frequent flushing of the 
> TLB. Is this also applied to  x86_64 cpus from other vendors other than AMD? 
> How can I verify it? It seems that the problem will have big impact for Dom0 
> performance, does it?

It does have - however Linux has made strides in minimizing the need for making 
TLB flushes and
also the major reason for doing up-calls is batched to minimize the amount the 
frequent usage of TLB flushes.

>     4. PVH Dom0 can improve the maintainability of pvops related code of Xen 
> as well as improve the performance of normal Dom0. I am wondering if there 
> are other aspects of performance PVH Dom0 can improve besides the 2 and 3 
> mentioned above.

Yes. We can use compound pages without the SWIOTLB finding the pages not being 
contingous. That
will improve network performance.
> [1] 
> https://blog.xenproject.org/2012/10/31/the-paravirtualization-spectrum-part-2-from-poles-to-a-spectrum/
> [2] http://www.brendangregg.com/blog/images/2014/xen-colors.png

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