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

[Julien Grall <julien@xen.org>]

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.

But, I was under the impression that there is plan to remove 
XEN_DOMCTL_max_vcpus. So it would only buy just a bit of time...

Cheers,

-- 
Julien Grall