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 4/19/2016 12:50 PM, Tian, Kevin wrote:
> > From: Yu, Zhang [mailto:yu.c.zhang@xxxxxxxxxxxxxxx]
> > Sent: Thursday, April 14, 2016 5:57 PM
> > > > > > > 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?
> I'm a bit worried about this change, since it's a fundamental EPT
> interface. Can we avoid supporting read emulation now?
Thanks for your reply, Kevin.
For XenGT, either waY would be fine. But I do not think changing
is_epte_present will introduce any bug.

Paul, do you have any preference?

Yu

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel