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

[Igor Mammedov]

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?
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)

> But why use 2G split instead of 3G?  There's only very little MMIO and
> no PCI hole (including no huge MMCONFIG BAR) on microvm.
> 
> Paolo
>