Message-ID: <1f3231c0-34b2-1e78-0bf0-f32d5b67811d@redhat.com>
Date:   Tue, 2 May 2023 17:09:06 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Matthew Rosato <mjrosato@...ux.ibm.com>,
        Jason Gunthorpe <jgg@...dia.com>
Cc:     Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Lorenzo Stoakes <lstoakes@...il.com>, linux-mm@...ck.org,
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        Theodore Ts'o <tytso@....edu>, Peter Xu <peterx@...hat.com>
Subject: Re: [PATCH v6 3/3] mm/gup: disallow FOLL_LONGTERM GUP-fast writing to
 file-backed mappings

On 02.05.23 15:56, Matthew Rosato wrote:
> On 5/2/23 9:50 AM, Jason Gunthorpe wrote:
>> On Tue, May 02, 2023 at 03:47:43PM +0200, David Hildenbrand wrote:
>>>> Eventually we want to implement a mechanism where we can dynamically pin in response to RPCIT.
>>>
>>> Okay, so IIRC we'll fail starting the domain early, that's good. And if we
>>> pin all guest memory (instead of small pieces dynamically), there is little
>>> existing use for file-backed RAM in such zPCI configurations (because memory
>>> cannot be reclaimed either way if it's all pinned), so likely there are no
>>> real existing users.
>>
>> Right, this is VFIO, the physical HW can't tolerate not having pinned
>> memory, so something somewhere is always pinning it.
> 
> I might have mis-explained above.
> 
> With iommufd nesting, we will pin everything upfront as a starting point.
> 
> The current usage of vfio type1 iommu for s390 does not pin the entirety of guest memory upfront, it happens as guest RPCITs occur / type1 mappings are made.

... so, after the domain started successfully on the libvirt/QEMU side ? :/

It would be great to confirm that. There might be a BUG in patch #2 (see 
my reply to patch #2) that might not allow you to reproduce it right now.

-- 
Thanks,

David / dhildenb

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