Re: Mapping memory into a domain

[Demi Marie Obenour <demiobenour@xxxxxxxxx>]

On 5/5/25 7:32 AM, Alejandro Vallejo wrote:
> I suppose this is still about multiplexing the GPU driver the way we
> last discussed at Xen Summit?
> 
> On Mon May 5, 2025 at 12:51 AM CEST, Demi Marie Obenour wrote:
>> What are the appropriate Xen internal functions for:
>>
>> 1. Turning a PFN into an MFN?
>> 2. Mapping an MFN into a guest?
>> 3. Unmapping that MFN from a guest?
> 
> The p2m is the single source of truth about such mappings.
> 
> This is all racy business. You want to keep the p2m lock for the full
> duration of whatever operation you wish do, or you risk another CPU
> taking it and pulling the rug under your feet at the most inconvenient
> time.
> 
> In general all this faff is hidden under way too many layers beneath
> copy_{to,from}_guest(). Other p2m manipulation high-level constructs
> that might do interesting things worth looking at may be 
> {map,unmap}_mmio_region()
> 
> Note that not every pfn has an associated mfn. Not even every valid pfn
> has necessarily an associated mfn (there's pod). And all of this is
> volatile business in the presence of a baloon driver or vPCI placing
> mmio windows over guest memory.

Can I check that POD is not in use?  

> In general anything up this alley would need a cohesive pair for
> map/unmap and a credible plan for concurrency and how it's all handled
> in conjunction with other bits that touch the p2m.

Is taking the p2m lock for the entire operation a reasonable approach
for concurrency?  Will this cause too much lock contention?

>> The first patch I am going to send with this information is a documentation
>> patch so that others do not need to figure this out for themselves.
>> I remember being unsure even after looking through the source code, which
>> is why I am asking here.
> 
> That's not surprising. There's per-arch stuff, per-p2mtype stuff,
> per-guesttype stuff. Plus madness like on-demand memory. It's no wonder
> such helpers don't exist and the general manipulations are hard to
> explain.

Is this a task that is only suitable for someone who has several years
experience working on Xen, or is it something that would make sense for
someone who is less experienced?
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)

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