> -----Original Message-----
> From: Jan Beulich <jbeul...@suse.com>
> Sent: 04 July 2019 10:19
> To: Paul Durrant <paul.durr...@citrix.com>; xen-devel@lists.xenproject.org
> Cc: Andrew Cooper <andrew.coop...@citrix.com>; George Dunlap
> <george.dun...@citrix.com>; Roger Pau
> Monne <roger....@citrix.com>; Wei Liu <w...@xen.org>
> Subject: Re: [PATCH v2] x86/HVM: p2m_ram_ro is incompatible with device
> pass-through
>
> On 03.07.2019 17:22, Paul Durrant wrote:
> >> -----Original Message-----
> >> From: Jan Beulich <jbeul...@suse.com>
> >> Sent: 03 July 2019 12:36
> >> To: xen-devel@lists.xenproject.org
> >> Cc: George Dunlap <george.dun...@citrix.com>; Paul Durrant
> >> <paul.durr...@citrix.com>; Andrew Cooper
> >> <andrew.coop...@citrix.com>; Wei Liu <w...@xen.org>; Roger Pau Monne
> >> <roger....@citrix.com>
> >> Subject: [PATCH v2] x86/HVM: p2m_ram_ro is incompatible with device
> >> pass-through
> >>
> >> The write-discard property of the type can't be represented in IOMMU
> >> page table entries. Make sure the respective checks / tracking can't
> >> race, by utilizing the domain lock. The other sides of the sharing/
> >> paging/log-dirty exclusion checks should subsequently perhaps also be
> >> put under that lock then.
> >>
> >> Take the opportunity and also convert neighboring bool_t to bool in
> >> struct hvm_domain.
> >>
> >> Signed-off-by: Jan Beulich <jbeul...@suse.com>
> >> ---
> >> v2: Don't set p2m_ram_ro_used when failing the request.
> >>
> >> --- a/xen/arch/x86/hvm/dm.c
> >> +++ b/xen/arch/x86/hvm/dm.c
> >> @@ -255,16 +255,33 @@ static int set_mem_type(struct domain *d
> >>
> >> mem_type = array_index_nospec(data->mem_type, ARRAY_SIZE(memtype));
> >>
> >> - if ( mem_type == HVMMEM_ioreq_server )
> >> + switch ( mem_type )
> >> {
> >> unsigned int flags;
> >>
> >> + case HVMMEM_ioreq_server:
> >> if ( !hap_enabled(d) )
> >> return -EOPNOTSUPP;
> >>
> >> /* Do not change to HVMMEM_ioreq_server if no ioreq server
> >> mapped. */
> >> if ( !p2m_get_ioreq_server(d, &flags) )
> >> return -EINVAL;
> >> +
> >> + break;
> >> +
> >> + case HVMMEM_ram_ro:
> >> + /* p2m_ram_ro can't be represented in IOMMU mappings. */
> >> + domain_lock(d);
> >> + if ( has_iommu_pt(d) )
> >> + rc = -EXDEV;
> >> + else
> >> + d->arch.hvm.p2m_ram_ro_used = true;
> >
> > Do we really want this to be a one-way trip? On the face of it, it
> > would seem that keeping a count of p2m_ram_ro entries would be more
> > desirable such that, once the last one is gone, devices can be
> > assigned again?
>
> Well, at this point I'm not really up to introducing accounting of
> the number of uses of p2m_ram_ro. This could be a further step to
> be done in the future, if necessary.
>
> > If not maybe it's time to go all the way and make iommu page table
> > construction part of domain create and then we simplify a lot of
> > code and we don't need any flag/refcount like this at all.
>
> I've said this before: I don't think it should be a requirement to
> know at the time of the creation of a VM whether it'll eventually
> have a pass-through device assigned. Furthermore you realize that
> this suggestion of yours is contrary to what you've said further up:
> This way you'd make the two things exclusive of one another without
> any recourse.
Yes, I realize the suggestions are contradictory. My point is that adding IOMMU
pages to a running domain is tricky and leads to issues like the one you are
trying to solve with the ram_ro_used flag.
The whole subsystem is in need of an overhaul anyway so I guess this band-aid
is ok for now.
Paul
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