Re: [Xen-devel] [PATCH v2] x86/HVM: p2m_ram_ro is incompatible with device pass-through

[Jan Beulich <JBeulich@xxxxxxxx>]

On 04.07.2019 11:35, Paul Durrant wrote:
>> -----Original Message-----
>> From: Jan Beulich <JBeulich@xxxxxxxx>
>> Sent: 04 July 2019 10:19
>> To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>; xen-devel@xxxxxxxxxxxxxxxxxxxx
>> Cc: Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>; George Dunlap 
>> <George.Dunlap@xxxxxxxxxx>; Roger Pau
>> Monne <roger.pau@xxxxxxxxxx>; Wei Liu <wl@xxxxxxx>
>> 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 <JBeulich@xxxxxxxx>
>>>> Sent: 03 July 2019 12:36
>>>> To: xen-devel@xxxxxxxxxxxxxxxxxxxx
>>>> Cc: George Dunlap <George.Dunlap@xxxxxxxxxx>; Paul Durrant 
>>>> <Paul.Durrant@xxxxxxxxxx>; Andrew Cooper
>>>> <Andrew.Cooper3@xxxxxxxxxx>; Wei Liu <wl@xxxxxxx>; Roger Pau Monne 
>>>> <roger.pau@xxxxxxxxxx>
>>>> 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 <jbeulich@xxxxxxxx>
>>>> ---
>>>> 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.

Thanks. I wonder whether I may translate this into R-b or A-b?

Jan
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