Re: [PATCH RFC 00/10] Xen balloon page compaction support

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 16/10/14 10:26, Ian Campbell wrote:
> On Wed, 2014-10-15 at 18:14 +0100, Andrew Cooper wrote:
>> On 15/10/14 18:00, Wei Liu wrote:
>>> On Wed, Oct 15, 2014 at 05:54:24PM +0100, Andrew Cooper wrote:
>>>> On 15/10/14 16:54, Wei Liu wrote:
>>>>> Hi all
>>>>>
>>>>> This is a prototype to make Xen balloon driver work with balloon page
>>>>> compaction. The goal is to reduce guest physical address space fragmentation
>>>>> introduced by balloon pages. Having guest physical address space as contiguous
>>>>> as possible is generally useful (because guest can have higher order pages), and
>>>>> it should also help improve HVM performance (provided that guest kernel knows
>>>>> how to use huge pages -- Linux has hugetlbfs and transparent huge page).
>>>> After you have defragmented guest physical memory, does Linux use
>>>> 2MB/1GB superpages more readily?
>>>>
>>> That's completely up to the guest. Having contiguous guest physical
>>> address space is prerequisite.
>>>
>>>> On what basis do you think this will improve HVM performance?  The HAP
>>>> tables still remain fragmented after ballooning.
>>>>
>>> That's the other side of this problem and orthogonal to this patch
>>> series. It should be fixed separately on the hypervisor side, presumably
>>> with similar mechanism to coalesce HAP table in hypervisor.
>> You can't rearrange the memory of any domain with passthrough, or any
>> subregion which is mapped by another domain.  Even if the underlying
>> pages are in order,
> That's fine. This work still improves things for plenty of other
> domains.
>
> I suspect with a suitable amount of cunning it might be possible to do
> this for domains with passthrough using smmu. Whether it is worth the
> time and effort (since I certain concede it won't be easy) I don't know.

x86 IOMMUs do not support restarable faults, and given the timing
constraints in the PCI spec, there is no obvious way to gain support
like this.

As a result, it is impossible for Xen to move a page, while maintaining
DMA read and write coherency for devices.

~Andrew