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Message-ID: <CAPcyv4hHjCpm4AnLz2SdtjNMasV182Cw-jA+Cv9DjmE1Fa26kA@mail.gmail.com>
Date: Fri, 20 Oct 2017 08:05:09 -0700
From: Dan Williams <dan.j.williams@...el.com>
To: Christoph Hellwig <hch@...radead.org>
Cc: Stefan Hajnoczi <stefanha@...il.com>,
Pankaj Gupta <pagupta@...hat.com>,
Kevin Wolf <kwolf@...hat.com>,
haozhong zhang <haozhong.zhang@...el.com>,
Jan Kara <jack@...e.cz>,
xiaoguangrong eric <xiaoguangrong.eric@...il.com>,
KVM list <kvm@...r.kernel.org>,
David Hildenbrand <david@...hat.com>,
linux-nvdimm <linux-nvdimm@...1.01.org>,
ross zwisler <ross.zwisler@...el.com>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
Qemu Developers <qemu-devel@...gnu.org>,
Linux MM <linux-mm@...ck.org>,
Stefan Hajnoczi <stefanha@...hat.com>,
Paolo Bonzini <pbonzini@...hat.com>,
Nitesh Narayan Lal <nilal@...hat.com>
Subject: Re: [Qemu-devel] [RFC 2/2] KVM: add virtio-pmem driver
On Fri, Oct 20, 2017 at 1:00 AM, Christoph Hellwig <hch@...radead.org> wrote:
> On Thu, Oct 19, 2017 at 11:21:26AM -0700, Dan Williams wrote:
>> The difference is that nvdimm_flush() is not mandatory, and that the
>> platform will automatically perform the same flush at power-fail.
>> Applications should be able to assume that if they are using MAP_SYNC
>> that no other coordination with the kernel or the hypervisor is
>> necessary.
>>
>> Advertising this as a generic Persistent Memory range to the guest
>> means that the guest could theoretically use it with device-dax where
>> there is no driver or filesystem sync interface. The hypervisor will
>> be waiting for flush notifications and the guest will just issue cache
>> flushes and sfence instructions. So, as far as I can see we need to
>> differentiate this virtio-model from standard "Persistent Memory" to
>> the guest and remove the possibility of guests/applications making the
>> wrong assumption.
>
> So add a flag that it is not. We already have the nd_volatile type,
> that is special. For now only in Linux, but I think adding this type
> to the spec eventually would be very useful for efficiently exposing
> directly mappable device to VM guests.
Right, that's the same recommendation I gave.
https://lists.gnu.org/archive/html/qemu-devel/2017-07/msg08404.html
...so maybe I'm misunderstanding your concern? It sounds like we're on
the same page.
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