Re: [Xen-devel] [PATCH] x86/HVM: p2m_ram_ro is incompatible with device pass-through

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> Sent: 02 May 2019 14:57
> To: Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>
> Cc: Paul Durrant <Paul.Durrant@xxxxxxxxxx>; Roger Pau Monne 
> <roger.pau@xxxxxxxxxx>; Wei Liu
> <wei.liu2@xxxxxxxxxx>; George Dunlap <George.Dunlap@xxxxxxxxxx>; xen-devel 
> <xen-
> devel@xxxxxxxxxxxxxxxxxxxx>
> Subject: Re: [PATCH] x86/HVM: p2m_ram_ro is incompatible with device 
> pass-through
> 
> >>> On 02.05.19 at 15:42, <andrew.cooper3@xxxxxxxxxx> wrote:
> > On 02/05/2019 14:16, Jan Beulich wrote:
> >>>>> On 02.05.19 at 14:59, <andrew.cooper3@xxxxxxxxxx> wrote:
> >>> On 02/05/2019 13:29, Jan Beulich wrote:
> >>>> --- a/xen/drivers/passthrough/pci.c
> >>>> +++ b/xen/drivers/passthrough/pci.c
> >>>> @@ -1450,17 +1450,36 @@ static int assign_device(struct domain *
> >>>>      if ( !iommu_enabled || !hd->platform_ops )
> >>>>          return 0;
> >>>>
> >>>> -    /* Prevent device assign if mem paging or mem sharing have been
> >>>> -     * enabled for this domain */
> >>>> -    if ( unlikely(d->arch.hvm.mem_sharing_enabled ||
> >>>> -                  vm_event_check_ring(d->vm_event_paging) ||
> >>>> +    domain_lock(d);
> >>>> +
> >>>> +    /*
> >>>> +     * Prevent device assignment if any of
> >>>> +     * - mem paging
> >>>> +     * - mem sharing
> >>>> +     * - the p2m_ram_ro type
> >>>> +     * - global log-dirty mode
> >>> XenServer has working live migration with GPUs, which this change would
> >>> regress.
> >>>
> >>> Behind the scenes, we combine Xen's logdirty bitmap with one provided by
> >>> the GPU, to ensure that all dirty updates are tracked.
> >> But this says nothing for the patch here.
> >
> > Yes it does.
> 
> Well, okay, then the wording of your reply plus me just adding
> a comment for a pre-existing check has mislead me.
> 
> > There is nothing inherent about global log-dirty mode which is
> > incompatible with passthrough when the IOMMU pagetables are not shared
> > with EPT.
> >
> >> As long as it doesn't work in the staging tree, it should be prevented.
> >
> > ... but it does work.
> 
> Note how (as said above) the patch does _not_ add any
> ->global_logdirty check, it merely adds a comment enumerating
> everything that's getting checked. I'm afraid I don't see how
> adding a comment to state how things are can regress anything.
> 
> The only check the patch adds is that of the new
> p2m_ram_ro_used flag.
> 

Actually, since global_logdirty is somewhat transient should we not also have a 
check to prevent p2m_ram_logdirty from being set for a domain with assigned 
devices and shared EPT?

  Paul

> Jan
> 


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