Re: xentrace, arm, hvm

[Julien Grall <julien.grall@linaro.org>]

On 02/03/2014 05:32 PM, Pavlo Suikov wrote:
> Hi,

Hello,

> has anyone used xentrace on arm with HVM domains? As far as I observe,
> it fails to map trace buffers from Xen restricted heap:

> xc_map_foreign_batch() call with DOMID_XEN permissions leads to
> xenmem_add_to_physmap_one() and then to rcu_lock_domain_by_any_id(),
> which fails to find DOMID_XEN in the domain hash (and it doesn't seem at
> all that dummy domains are added to this hash). Actually I don't see how
> this could work at all since there are no obvious checks for either arch
> or domain type (PV or HVM) along the way.

After a quick look to Xen, it seems that xentrace was only working for
x86 PV (the issue will be the same on PVH). It was working because PV
domain uses mmu_update hypercall. In this function, x86 has a specific
case for DOMID_XEN (see get_pg_owner).

To support xentrace on ARM, we will need at least:
  - to replace rcu_lock_domain_by_any_id() by a a similar function
  - to add stubs for trace in arm code

BTW, when I tried xentrace I have this following error in kernel log
messages:
Failed to map pfn to mfn rc:0:-3 pfn:1e9d0 mfn:fdfbe
xen_privcmd: unable to unmap MFN range: leaking 1 pages. rc=-2

Is it normal that Linux is trying to unmap a page that has failed to map
ealier?

Sincerely yours,

-- 
Julien Grall