Re: [PATCH v3 2/5] xentrace: Memory/Page Mapping support for DOMID_XEN on ARM

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 08/04/16 16:49, Jan Beulich wrote:
>>>> On 08.04.16 at 12:42, <julien.grall@arm.com> wrote:
>> On 04/04/16 19:48, Benjamin Sanda wrote:
>>> +    else
>>> +    {
>>> +        /* retrieve the page to determine read/write or read only mapping */
>>> +        mfn = paddr >> PAGE_SHIFT;
>>> +        if (mfn_valid(mfn))
>>> +        {
>>> +            page = mfn_to_page(mfn);
>>> +            *t = (page->u.inuse.type_info == PGT_writable_page ?
>>> +                                p2m_ram_rw : p2m_ram_ro);
>> Unfortunately, xenmem_add_to_physmap_one will ignore the return type and 
>> will always map using the type p2m_map_foreign. I would introduce
>> a new type p2m_map_foreign_ro to allow read-only foreign mapping.
>>
>> I've looked at the x86 code (p2m_add_foreign) and I haven't been able to 
>> find how the page will be mapped read-only in the guest P2M. 
>> get_page_from_gfn will always return p2m_raw_rw for DOMID_XEN as it's a 
>> non translated domain.
>>
>> Andrew and Jan, do you know how this is supposed to work when xentrace 
>> is used in a HVM domain? Does x86 Xen always mapped Read-Write the page?
> I don't think that case is being taken care of right now: xentrace
> is to be used by privileged guests only anyway, and the only
> HVM-like privileged guest would be a PVHv1 Dom0 (which likely
> no-one cared about to make work with xentrace so far).

Answer to questions of the form "Has anyone considered $X for a
privileged HVM domain on x86" are almost always "No".

The real question is whether the domain making the mapping needs to
write into the pages or not.  If xentrace has to update shared pointers,
then it needs to be rw.  If it simply consumes the data without any
backwards notification, then it should be ro.

~Andrew

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