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>]

On 4/12/2016 11:08 PM, Jan Beulich wrote:
> > > > "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?
> Yes.
>
> > > > > 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 
> before
> deciding to go such a route.
Thanks, Jan. :)
Jun & Kevin, any suggestions?

Yu

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