Re: [PATCH v3 14/22] microvm: use 2G split unconditionally

[Paolo Bonzini]

On 27/05/20 16:26, Igor Mammedov wrote:
> On Wed, 27 May 2020 15:06:28 +0200
> Paolo Bonzini <address@hidden> wrote:
> 
>> On 27/05/20 14:25, Igor Mammedov wrote:
>>>>   (2) we loose a gigabyte page.  
>>>           I'm not sure waht exactly we loose in this case.
>>>           Lets assume we allocating guest 5G of continuous RAM using 1G 
>>> huge pages,
>>>           in this case I'd think that on host side MMIO overlay won't 
>>> affect RAM blob
>>>           on guest side pagetables will be fragmented due to MMIO holes, 
>>> but guest still
>>>           could use huge pages smaller ones in fragmented area and 1G where 
>>> there is no fragmentation.  
>>
>> Access to the 3G-4G area would not be able to use 1G EPT pages.
> Could it use 2Mb pages instead of 1Gb?

Yes, probably a mix of 2 MiB pages and 4 KiB pages around the memslot
splits.

> Do we really care about 1 gigabyte huge page in microvm intended usecase?
> (fast starting VMs for microservices like FaaS, which unlikely would use much 
> memory to begin with)

I honestly don't think it's measurable, but at least in theory we care
because such workloads could have more TLB misses (relative to the
execution time) than long-lasting VMs.

Paolo