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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> 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.

Jan



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