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

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.



Oleksandr Tyshchenko



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