linux-kernel - Re: [PATCH v4 2/2] kvm: Use huge pages for DAX-backed files

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Date:   Thu, 12 Dec 2019 15:08:04 -0500
From:   Barret Rhoden <brho@...gle.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        David Hildenbrand <david@...hat.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Alexander Duyck <alexander.h.duyck@...ux.intel.com>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        X86 ML <x86@...nel.org>, KVM list <kvm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "Zeng, Jason" <jason.zeng@...el.com>,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v4 2/2] kvm: Use huge pages for DAX-backed files

On 12/12/19 2:48 PM, Dan Williams wrote:
> On Thu, Dec 12, 2019 at 11:16 AM Barret Rhoden <brho@...gle.com> wrote:
>>
>> On 12/12/19 12:37 PM, Dan Williams wrote:
>>> Yeah, since device-dax is the only path to support longterm page
>>> pinning for vfio device assignment, testing with device-dax + 1GB
>>> pages would be a useful sanity check.
>>
>> What are the issues with fs-dax and page pinning?  Is that limitation
>> something that is permanent and unfixable (by me or anyone)?
> 
> It's a surprisingly painful point of contention...

Thanks for the info; I'll check out those threads.

[snip]

>> I'd like to put a lot more in a DAX/pmem region than just a guest's
>> memory, and having a mountable filesystem would be extremely convenient.
> 
> Why would page pinning be involved in allowing the guest to mount a
> filesystem on guest-pmem? That already works today, it's just the
> device-passthrough that causes guest memory to be pinned indefinitely.

I'd like to mount the pmem filesystem on the *host* and use its files 
for the guest's memory.  So far I've just been making an ext4 FS on 
/dev/pmem0 and creating a bunch of files in the FS.  Some of the files 
are the guest memory: one file for each VM.  Other files are just 
metadata that the host uses.

That all works right now, but I'd also like to use VFIO with the guests.

Thanks,

Barret