RE: [PATCH] x86/hvm: set 'ipat' in EPT for special pages

[Paul Durrant <xadimgnik@gmail.com>]

> -----Original Message-----
> From: Andrew Cooper <andrew.cooper3@citrix.com>
> Sent: 31 July 2020 12:21
> To: Paul Durrant <paul@xen.org>; xen-devel@lists.xenproject.org
> Cc: Paul Durrant <pdurrant@amazon.com>; Jan Beulich <jbeulich@suse.com>; Wei Liu <wl@xen.org>; Roger
> Pau Monné <roger.pau@citrix.com>
> Subject: Re: [PATCH] x86/hvm: set 'ipat' in EPT for special pages
> 
> On 31/07/2020 11:46, Paul Durrant wrote:
> > From: Paul Durrant <pdurrant@amazon.com>
> >
> > All non-MMIO ranges (i.e those not mapping real device MMIO regions) that
> > map valid MFNs are normally marked MTRR_TYPE_WRBACK and 'ipat' is set. Hence
> > when PV drivers running in a guest populate the BAR space of the Xen Platform
> > PCI Device with pages such as the Shared Info page or Grant Table pages,
> > accesses to these pages will be cachable.
> >
> > However, should IOMMU mappings be enabled be enabled for the guest then these
> > accesses become uncachable. This has a substantial negative effect on I/O
> > throughput of PV devices. Arguably PV drivers should bot be using BAR space to
> > host the Shared Info and Grant Table pages but it is currently commonplace for
> > them to do this and so this problem needs mitigation. Hence this patch makes
> > sure the 'ipat' bit is set for any special page regardless of where in GFN
> > space it is mapped.
> >
> > NOTE: Clearly this mitigation only applies to Intel EPT. It is not obvious
> >       that there is any similar mitigation possible for AMD NPT. Downstreams
> >       such as Citrix XenServer have been carrying a patch similar to this for
> >       several releases though.
> 
> https://github.com/xenserver/xen.pg/blob/XS-8.2.x/master/xen-override-caching-cp-26562.patch
> 
> (Yay for internal ticket references escaping into the wild.)
> 

:-)

> 
> However, it is very important to be aware that this is just papering
> over the problem, and it will cease to function as soon as we get MKTME
> support.  When we hit that point, iPAT cannot be used, as it will cause
> data corruption in guests.
> 
> The only correct way to fix this is to not (mis)use BAR space for RAM
> mappings.
> 

Oh yes, t
his is only a mitigation. I believe Roger is working on a mechanism for guests to query for non-populated RAM space, which would be suitable for use by PV drivers.

  Paul

> ~Andrew