Re: [PATCH RFC 0/5] mm/gup: Introduce exclusive GUP pinning

David Hildenbrand
2024-06-21T08:02:14Z

On 21.06.24 09:32, Quentin Perret wrote:
> On Thursday 20 Jun 2024 at 20:18:14 (-0300), Jason Gunthorpe wrote:
>> On Thu, Jun 20, 2024 at 03:47:23PM -0700, Elliot Berman wrote:
>>> On Thu, Jun 20, 2024 at 11:29:56AM -0300, Jason Gunthorpe wrote:
>>>> On Thu, Jun 20, 2024 at 04:01:08PM +0200, David Hildenbrand wrote:
>>>>> Regarding huge pages: assume the huge page (e.g., 1 GiB hugetlb) is shared,
>>>>> now the VM requests to make one subpage private.
>>>>
>>>> I think the general CC model has the shared/private setup earlier on
>>>> the VM lifecycle with large runs of contiguous pages. It would only
>>>> become a problem if you intend to to high rate fine granual
>>>> shared/private switching. Which is why I am asking what the actual
>>>> "why" is here.
>>>>
>>>
>>> I'd let Fuad comment if he's aware of any specific/concrete Anrdoid
>>> usecases about converting between shared and private. One usecase I can
>>> think about is host providing large multimedia blobs (e.g. video) to the
>>> guest. Rather than using swiotlb, the CC guest can share pages back with
>>> the host so host can copy the blob in, possibly using H/W accel. I
>>> mention this example because we may not need to support shared/private
>>> conversions at granularity finer than huge pages.
>>
>> I suspect the more useful thing would be to be able to allocate actual
>> shared memory and use that to shuffle data without a copy, setup much
>> less frequently. Ie you could allocate a large shared buffer for video
>> sharing and stream the video frames through that memory without copy.
>>
>> This is slightly different from converting arbitary memory in-place
>> into shared memory. The VM may be able to do a better job at
>> clustering the shared memory allocation requests, ie locate them all
>> within a 1GB region to further optimize the host side.
>>
>>> Jason, do you have scenario in mind? I couldn't tell if we now had a
>>> usecase or are brainstorming a solution to have a solution.
>>
>> No, I'm interested in what pKVM is doing that needs this to be so much
>> different than the CC case..
>
> The underlying technology for implementing CC is obviously very
> different (MMU-based for pKVM, encryption-based for the others + some
> extra bits but let's keep it simple). In-place conversion is inherently
> painful with encryption-based schemes, so it's not a surprise the
> approach taken in these cases is built around destructive conversions as
> a core construct. But as Elliot highlighted, the MMU-based approach
> allows for pretty flexible and efficient zero-copy, which we're not
> ready to sacrifice purely to shoehorn pKVM into a model that was
> designed for a technology that has very different set of constraints.
> A private->shared conversion in the pKVM case is nothing more than
> setting a PTE in the recipient's stage-2 page-table.
>
> I'm not at all against starting with something simple and bouncing via
> swiotlb, that is totally fine. What is _not_ fine however would be to
> bake into the userspace API that conversions are not in-place and
> destructive (which in my mind equates to 'you can't mmap guest_memfd
> pages'). But I think that isn't really a point of disagreement these
> days, so hopefully we're aligned.
>
> And to clarify some things I've also read in the thread, pKVM can
> handle the vast majority of faults caused by accesses to protected
> memory just fine. Userspace accesses protected guest memory? Fine,
> we'll SEGV the userspace process. The kernel accesses via uaccess
> macros? Also fine, we'll fail the syscall (or whatever it is we're
> doing) cleanly -- the whole extable machinery works OK, which also
> means that things like load_unaligned_zeropad() keep working as-is.
> The only thing pKVM does is re-inject the fault back into the kernel
> with some extra syndrome information it can figure out what to do by
> itself.
>
> It's really only accesses via e.g. the linear map that are problematic,
> hence the exclusive GUP approach proposed in the series that tries to
> avoid that by construction. That has the benefit of leaving
> guest_memfd to other CC solutions that have more things in common. I
> think it's good for that discussion to happen, no matter what we end up
> doing in the end.

Thanks for the information. IMHO we really should try to find a common
ground here, and FOLL_EXCLUSIVE is likely not it :)

Thanks for reviving this discussion with your patch set!

pKVM is interested in in-place conversion, I believe there are valid use
cases for in-place conversion for TDX and friends as well (as discussed,
I think that might be a clean way to get huge/gigantic page support in).

This implies the option to:

1) Have shared+private memory in guest_memfd
2) Be able to mmap shared parts
3) Be able to convert shared<->private in place

and later in my interest

4) Have huge/gigantic page support in guest_memfd with the option of
converting individual subpages

We might not want to make use of that model for all of CC -- as you
state, sometimes the destructive approach might be better performance
wise -- but having that option doesn't sound crazy to me (and maybe
would solve real issues as well).

After all, the common requirement here is that "private" pages are not
mapped/pinned/accessible.

Sure, there might be cases like "pKVM can handle access to private pages
in user page mappings", "AMD-SNP will not crash the host if writing to
private pages" but there are not factors that really make a difference
for a common solution.

private memory: not mapped, not pinned
shared memory: maybe mapped, maybe pinned
granularity of conversion: single pages

Anything I am missing?

--
Cheers,

David / dhildenb