Re: [for-4.15][PATCH v2 1/5] xen/x86: p2m: Don't map the special pages in the IOMMU page-tables

[Jan Beulich <jbeulich@xxxxxxxx>]

On 10.02.2021 12:48, Julien Grall wrote:
> 
> 
> On 10/02/2021 11:45, Jan Beulich wrote:
>> On 10.02.2021 12:40, Julien Grall wrote:
>>> On 10/02/2021 11:38, Jan Beulich wrote:
>>>> On 10.02.2021 12:34, Roger Pau Monné wrote:
>>>>> On Wed, Feb 10, 2021 at 12:10:09PM +0100, Jan Beulich wrote:
>>>>>> On 10.02.2021 09:29, Roger Pau Monné wrote:
>>>>>>> I get the feeling this is just papering over an existing issue instead
>>>>>>> of actually fixing it: IOMMU page tables need to be properly freed
>>>>>>> during early failure.
>>>>>>
>>>>>> I take a different perspective: IOMMU page tables shouldn't
>>>>>> get created (yet) at all in the course of
>>>>>> XEN_DOMCTL_createdomain - this op is supposed to produce an
>>>>>> empty container for a VM.
>>>>>
>>>>> The same would apply for CPU page-tables then, yet they seem to be
>>>>> created and populating them (ie: adding the lapic access page) doesn't
>>>>> leak such entries, which points at an asymmetry. Either we setup both
>>>>> tables and handle freeing them properly, or we set none of them.
>>>>
>>>> Where would CPU page tables get created from at this early stage?
>>>
>>> When mapping the APIC page in the P2M. I don't think you can get away
>>> with removing it completely.
>>
>> It doesn't need putting in the p2m this early. It would be quite
>> fine to defer this until e.g. the first vCPU gets created.
> 
> It feels wrong to me to setup a per-domain mapping when initializing the 
> first vCPU.

Then we could do it even later. Any time up to when the domain
would first get unpaused would do.

Jan