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 5:40 PM, Paul Durrant wrote:
> > -----Original Message-----
> > From: Yu, Zhang [mailto:yu.c.zhang@xxxxxxxxxxxxxxx]
> > Sent: 19 April 2016 10:27
> > To: Paul Durrant; Kevin Tian; Jan Beulich; Nakajima, Jun
> > Cc: Keir (Xen.org); Andrew Cooper; Tim (Xen.org); George Dunlap; xen-
> > devel@xxxxxxxxxxxxx; Lv, Zhiyuan
> > Subject: Re: [Xen-devel] [PATCH v2 3/3] x86/ioreq server: Add HVMOP to
> > map guest ram with p2m_ioreq_server to an ioreq server
> >
> >
> >
> > On 4/19/2016 4:46 PM, Paul Durrant wrote:
> > > > -----Original Message-----
> > > > From: Tian, Kevin [mailto:kevin.tian@xxxxxxxxx]
> > > > Sent: 19 April 2016 05:51
> > > > To: Yu, Zhang; Jan Beulich; Paul Durrant; Nakajima, Jun
> > > > Cc: Andrew Cooper; George Dunlap; Lv, Zhiyuan; xen-
> > devel@xxxxxxxxxxxxx;
> > > > Keir (Xen.org); Tim (Xen.org)
> > > > Subject: RE: [Xen-devel] [PATCH v2 3/3] x86/ioreq server: Add HVMOP to
> > > > map guest ram with p2m_ioreq_server to an ioreq server
> > > >
> > > > > 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?
> > > I'm happy to drop the implementation of read emulation for the moment
> > to keep things simple, as long as it is kept in the interface.
> > >
> > Thanks, Paul.
> > So what's the benefit to keep the read in the interface, if we can
> > not support its emulation?
> Well, if it's in the interface then support can be added in future. If it's not 
> in the interface then it needs to be added later and that makes things more 
> awkward compatibility-wise.
>
>    Paul
But if we need to support read emulation in the future, we'll have to 
clear bit0:2 in ept pte, which is the same situation we are facing
now. So why do we not support it now?

Sorry for seeming so stubborn, I just want to have some more convincing
explanations. :)

Yu

> > It could be easier to change the is_epte_present definition than
> > to remove the read emulation code, but either way would not be
> > difficult.
> >
> > Yu
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