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

[Igor Mammedov]

On Wed, 27 May 2020 16:26:46 +0200
Igor Mammedov <address@hidden> 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?
> 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)

my interest in having single memory region, is in possibility of drop in 
conversion to [nv|pc-dimm] later on
without breaking ABI. (I'm not sure that we actually need it though)


> > 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
> >   
> 
>