Re: [Xen-devel] [RFC PATCH 2/2] x86/mm: Add mem access rights to NPT

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: George Dunlap [mailto:george.dunlap@xxxxxxxxxx]
> Sent: 27 September 2018 11:38
> To: Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>; xen-
> devel@xxxxxxxxxxxxxxxxxxxx
> Cc: Isaila Alexandru <aisaila@xxxxxxxxxxxxxxx>; Jan Beulich
> <jbeulich@xxxxxxxx>; Tim (Xen.org) <tim@xxxxxxx>; Tamas K Lengyel
> <tamas.lengyel@xxxxxxxxxxxx>; Paul Durrant <Paul.Durrant@xxxxxxxxxx>;
> Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>; Suravee Suthikulpanit
> <suravee.suthikulpanit@xxxxxxx>; Brian Woods <brian.woods@xxxxxxx>; Boris
> Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
> Subject: Re: [RFC PATCH 2/2] x86/mm: Add mem access rights to NPT
> 
> On 09/26/2018 06:22 PM, Andrew Cooper wrote:
> > On 26/09/18 17:47, George Dunlap wrote:
> >> From: Isaila Alexandru <aisaila@xxxxxxxxxxxxxxx>
> >>
> >> This patch adds access control for NPT mode.
> >>
> >> There aren’t enough extra bits to store the access rights in the NPT
> p2m
> >> table, so we add a radix tree to store extra information.
> >
> > I'm sorry to re-open this argument, but why?
> >
> > ISTR there being some argument based on pagetable sharing with the
> > IOMMU, but that doesn't work at the moment and can't reasonably be made
> > to work.  For one, attempting to use pt sharing will break as soon as
> > you try and DMA to a mapped grant.
> >
> > I'm disinclined to let a broken vestigial feature get in the way of real
> > improvements.
> >
> > Beyond that, an NPT PTE has basically the same number of software
> > available bits as an EPT PTE.
> >
> > Am I missing anything?
> 
> Wow -- looks like IOMMU/p2m sharing has been disabled unconditionally
> since 2014.  If nobody has complained since then, that seems like a good
> enough reason to me to rip it out.
> 
> Suravee / Brian / Boris -- any opinions?
> 
> The main reason to go with the 'extra bits' solution rather than the
> 'rip out iommu/p2m sharing' solution is because people have been
> prognosticating for years that we would be running out of bits and need
> more at some point in the future.  I thought Paul, for instance, might
> have a use for the extra bits.  But I'm happy to wait until such time as
> we need it and then fish this patch out of the mail archives.
> 

The main angle I had was to have a more generic page-to-type mapping such that 
it would be suitable to allow steering of accesses to certain pages to distinct 
IOREQ servers.

  Paul

>  -George
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