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Re: [Xen-devel] [PATCH v2 3/3] x86/ioreq server: Add HVMOP to map guest ram with p2m_ioreq_server to an ioreq server

>>> "Yu, Zhang" <yu.c.zhang@xxxxxxxxxxxxxxx> 04/12/16 11:47 AM >>>
>On 4/12/2016 12:31 AM, Jan Beulich wrote:
>>>>> On 11.04.16 at 13:14, <yu.c.zhang@xxxxxxxxxxxxxxx> wrote:
>>> On 4/9/2016 6:28 AM, Jan Beulich wrote:
>>>>>>> On 31.03.16 at 12:53, <yu.c.zhang@xxxxxxxxxxxxxxx> wrote:
>>>>> +        if ( s->id == id )
>>>>> +        {
>>>>> +            rc = p2m_set_ioreq_server(d, flags, s);
>>>>> +            if ( rc == 0 )
>>>>> +                gdprintk(XENLOG_DEBUG, "%u %s type 
>>>>> HVMMEM_ioreq_server.\n",
>>>>> +                         s->id, (flags != 0) ? "mapped to" : "unmapped 
>>>>> from");
>>>> Why gdprintk()? I don't think the current domain is of much
>>>> interest here. What would be of interest is the subject domain.
>>> s->id is not the domain_id, but id of the ioreq server.
>> That's understood. But gdprintk() itself logs the current domain,
>> which isn't as useful as the subject one.
>Oh, I see. So the correct routine here should be dprintk(), right?


>>>> And with all three bits now possibly being clear, aren't we risking the
>>>> entries to be mis-treated as not-present ones?
>>> Hah. You got me. Thanks! :)
>>> Now I realized it would be difficult if we wanna to emulate the read
>>> operations for HVM. According to Intel mannual, entry->r is to be
>>> cleared, so should entry->w if we do not want ept misconfig. And
>>> with both read and write permissions being forbidden, entry->x can be
>>> set only on processors with EXECUTE_ONLY capability.
>>> To avoid any entry to be mis-treated as not-present. We have several
>>> solutions:
>>> a> do not support the read emulation for now - we have no such usage
>>> case;
>>> b> add the check of p2m_t against p2m_ioreq_server in is_epte_present -
>>> a bit weird to me.
>>> Which one do you prefer? or any other suggestions?
>> That question would also need to be asked to others who had
>> suggested supporting both. I'd be fine with a, but I also don't view
>> b as too awkward.
>According to Intel mannual, an entry is regarded as not present, if
>bit0:2 is 0. So adding a p2m type check in is_epte_present() means we
>will change its semantics, if this is acceptable(with no hurt to
>hypervisor). I'd prefer option b>

Perhaps time for the VMX maintainers to chime in - such a change is acceptable
only if it doesn't result in changed hardware behavior. I can't think of any 
such off
the top of my head, but this really should be confirmed by the maintainers 
deciding to go such a route.


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