Re: Memory Layout on Dom0 in PV

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 16/04/2021 15:58, Charles Gonçalves wrote:

Hello Guys,

Does memory on Dom0 also mapped to gpfn or it is mapped directly to mfn? 

 If mapped to gpfn, how can I access its p2m mapping?

I'm trying to use the xen-mfndump but it is not working with dom0

./xen-mfndump dump-p2m 0                                                                          
xc: error: Could not map the shared info frame (MFN 0xddfe9) (3 = No such process): Internal error
xc: error: Could not map domain 0 memory information: Internal error

The problem that I'm wanting to solve is:

Given a va in a process on dom0, I can get its physical representation (mfn or gpfn depending on how this works on PV) using /proc/pid/pagemap and with this I can access its real mfn.

This works perfectly with domU but not with dom0, why?

    There are a lot of operations you're not permitted to perform on
    yourself, to prevent problems.
    
    I think in this case, Xen is rejecting dom0's attempt to foreign map
    itself (although -EINVAL would be better than -ESRCH to fail with,
    IMO), because otherwise we get into reference counting issues IIRC.
    
    You'll probably have an easier time asking the dom0 kernel to dump
    the p2m.  After all, its a kernel-owned datastructure, not a
    Xen-owned structure.
    
    ~Andrew