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

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Wed, 6 Oct 2021, Oleksandr wrote:
> 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.

Two more things about this being ARM-specific.

Even if x86 was affected exactly by the same problem, the code to expose
the safe memory ranges to DomU is arch-specific (currently device tree.)

Also the code to calculate the safe memory ranges is arch-specific as it
depends on the DomU memory layout which is arch-specific.