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

[Igor Mammedov]

On Tue, 26 May 2020 06:48:39 +0200
Gerd Hoffmann <address@hidden> wrote:

> > > Well, I think we can (should) drop max-ram-below-4g too.  There is
> > > no reason to use that with microvm, other that shooting yourself into
> > > the foot (by making mmio overlap with ram).
> > > 
> > > With that being gone too there isn't much logic left ...  
> > 
> > I wonder if we need 2G split for microvm at all?
> > Can we map 1 contiguous big blob from 0 GPA and overlay bios & other x86 
> > TOLUD stuff?  
> 
> I think it would work, but it has some drawbacks:
> 
>   (1) we loose a bit of memory.
          it's probably not a big enough to care about, we do similar ovarlay 
mapping on pc/q35
          at the beginning of RAM
>   (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.

>   (3) we wouldn't have guard pages (unused address space) between
>       between ram and mmio space.
           if it's holes' mmio,then do we really need them (access is going to 
be terminated
           either in always valid RAM or in valid mmio hole)?
> 
> take care,
>   Gerd
> 
>