Re: [PATCH] VMX: use a single, global APIC access page

[Jan Beulich <jbeulich@xxxxxxxx>]

On 11.02.2021 12:16, Roger Pau Monné wrote:
> On Thu, Feb 11, 2021 at 11:36:59AM +0100, Jan Beulich wrote:
>> On 11.02.2021 09:45, Roger Pau Monné wrote:
>>> On Wed, Feb 10, 2021 at 05:48:26PM +0100, Jan Beulich wrote:
>>>> --- a/xen/include/asm-x86/p2m.h
>>>> +++ b/xen/include/asm-x86/p2m.h
>>>> @@ -935,6 +935,9 @@ static inline unsigned int p2m_get_iommu
>>>>          flags = IOMMUF_readable;
>>>>          if ( !rangeset_contains_singleton(mmio_ro_ranges, mfn_x(mfn)) )
>>>>              flags |= IOMMUF_writable;
>>>> +        /* VMX'es APIC access page is global and hence has no owner. */
>>>> +        if ( mfn_valid(mfn) && !page_get_owner(mfn_to_page(mfn)) )
>>>> +            flags = 0;
>>>
>>> Is it fine to have this page accessible to devices if the page tables
>>> are shared between the CPU and the IOMMU?
>>
>> No, it's not, but what do you do? As said elsewhere, devices
>> gaining more access than is helpful is the price we pay for
>> being able to share page tables. But ...
> 
> I'm concerned about allowing devices to write to this shared page, as
> could be used as an unintended way to exchange information between
> domains?

Well, such an abuse would be possible, but it wouldn't be part
of an ABI and hence could break at any time. Similarly I
wouldn't consider it an information leak if a guest abused
this.

>>> Is it possible for devices to write to it?
>>
>> ... thinking about it - they would be able to access it only
>> when interrupt remapping is off. Otherwise the entire range
>> 0xFEExxxxx gets treated differently altogether by the IOMMU,
> 
> Now that I think of it, the range 0xFEExxxxx must always be special
> handled for device accesses, regardless of whether interrupt remapping
> is enabled or not, or else they won't be capable of delivering MSI
> messages?

I don't think I know how exactly chipsets handle MSI in this
case, but yes, presumably these accesses need to be routed a
different path even in that case.

> So I assume that whatever gets mapped in the IOMMU pages tables at
> 0xFEExxxxx simply gets ignored?

This would be the implication, aiui.

> Or else mapping the lapic access page when using shared page tables
> would have prevented CPU#0 from receiving MSI messages.

I guess I don't understand this part. In particular I see
nothing CPU#0 specific here.

Jan