Re: [PATCH V5 2/3] libxl/arm: Add handling of extended regions for DomU

[Oleksandr <olekstysh@xxxxxxxxx>]

On 06.10.21 14:34, Ian Jackson wrote:

Hi Ian

> Oleksandr Tyshchenko writes ("[PATCH V5 2/3] libxl/arm: Add handling of extended 
> regions for DomU"):
> > The extended region (safe range) is a region of guest physical
> > address space which is unused and could be safely used to create
> > grant/foreign mappings instead of wasting real RAM pages from
> > the domain memory for establishing these mappings.
> Please forgive me for asking this question now, but: why is this
> ARM-specific ?

Sorry, I can't say for sure which x86 mode also suffers from that. I 
might be wrong, but as I understand that x86 in PVH (and HVM?) mode uses 
unpopulated memory ranges (which are unused from Linux PoV, actually 
everything not yet allocated or reserved from "iomem_resource") to 
create foreign/grant mappings. So the real RAM pages are not ballooned 
out to get an physical address space to create these mappings. The 
problem is that we cannot follow Linux advise which memory ranges are 
unused on Arm for several reasons, this is why this patch series makes 
the hypervisor to start allocating and exposing these ranges.
> Ian.
--
Regards,

Oleksandr Tyshchenko