Re: [PATCH 5/5] vtd: Move intremap table to xenheap

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 05/08/2026 10:44 am, Jan Beulich wrote:
> On 29.07.2026 11:59, Teddy Astie wrote:
>> Interrupt remapping entries often needs to be accessed, and we're creating
>> pointers to it on demand, which brings a lot of complexity (e.g
>> GET_IREMAP_ENTRY() macro), move it to xenheap such that it's persistently
>> mapped and we won't have to worry about mapping and unmapping individual
>> intremap table pages.
> Afaic: No movement from domheap to xenheap except for _very_ good reasons.
> For the case here that is - maybe establish a permanent mapping using
> vmap(), but no change in where the memory is to come from. Whether such a
> permanent mapping is really worthwhile may also want supporting by numbers.
> You say "often", but you don't qualify / quantify this any further.

To expand on the "why" a bit more.

For systems with all RAM below the 4T boundary, domheap and xenheap are
equivalent.  We have 5T of directmap, but xenheap allocations have a
width restriction which is a power-of-2.

For systems with any RAM above the 4T boundary, you can't have xenheap
allocations be NUMA-local for all NUMA nodes.


As for "often", the IRTEs are modified every time a vCPU moves to a
different PCPU, because the target addresses need updating.  While it
probably doesn't matter much today, in the context of ASI it's something
which would want mapping permanently, rather than on-demand.

~Andrew