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Re: [Qemu-devel] KVM "fake DAX" flushing interface - discussion
From: |
David Hildenbrand |
Subject: |
Re: [Qemu-devel] KVM "fake DAX" flushing interface - discussion |
Date: |
Thu, 18 Jan 2018 17:53:03 +0100 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.5.2 |
On 24.11.2017 13:40, Pankaj Gupta wrote:
>
> Hello,
>
> Thank you all for all the useful suggestions.
> I want to summarize the discussions so far in the
> thread. Please see below:
>
>>>>
>>>>> We can go with the "best" interface for what
>>>>> could be a relatively slow flush (fsync on a
>>>>> file on ssd/disk on the host), which requires
>>>>> that the flushing task wait on completion
>>>>> asynchronously.
>>>>
>>>>
>>>> I'd like to clarify the interface of "wait on completion
>>>> asynchronously" and KVM async page fault a bit more.
>>>>
>>>> Current design of async-page-fault only works on RAM rather
>>>> than MMIO, i.e, if the page fault caused by accessing the
>>>> device memory of a emulated device, it needs to go to
>>>> userspace (QEMU) which emulates the operation in vCPU's
>>>> thread.
>>>>
>>>> As i mentioned before the memory region used for vNVDIMM
>>>> flush interface should be MMIO and consider its support
>>>> on other hypervisors, so we do better push this async
>>>> mechanism into the flush interface design itself rather
>>>> than depends on kvm async-page-fault.
>>>
>>> I would expect this interface to be virtio-ring based to queue flush
>>> requests asynchronously to the host.
>>
>> Could we reuse the virtio-blk device, only with a different device id?
>
> As per previous discussions, there were suggestions on main two parts of the
> project:
>
> 1] Expose vNVDIMM memory range to KVM guest.
>
> - Add flag in ACPI NFIT table for this new memory type. Do we need NVDIMM
> spec
> changes for this?
>
> - Guest should be able to add this memory in system memory map. Name of
> the added memory in
> '/proc/iomem' should be different(shared memory?) than persistent memory
> as it
> does not satisfy exact definition of persistent memory (requires an
> explicit flush).
>
> - Guest should not allow 'device-dax' and other fancy features which are
> not
> virtualization friendly.
>
> 2] Flushing interface to persist guest changes.
>
> - As per suggestion by ChristophH (CCed), we explored options other then
> virtio like MMIO etc.
> Looks like most of these options are not use-case friendly. As we want
> to do fsync on a
> file on ssd/disk on the host and we cannot make guest vCPU's wait for
> that time.
>
> - Though adding new driver(virtio-pmem) looks like repeated work and not
> needed so we can
> go with the existing pmem driver and add flush specific to this new
> memory type.
I'd like to emphasize again, that I would prefer a virtio-pmem only
solution.
There are architectures out there (e.g. s390x) that don't support
NVDIMMs - there is no HW interface to expose any such stuff.
However, with virtio-pmem, we could make it work also on architectures
not having ACPI and friends.
>
> - Suggestion by Paolo & Stefan(previously) to use virtio-blk makes sense
> if just
> want a flush vehicle to send guest commands to host and get reply after
> asynchronous
> execution. There was previous discussion [1] with Rik & Dan on this.
>
> [1] https://lists.gnu.org/archive/html/qemu-devel/2017-07/msg08373.html
>
> Is my understanding correct here?
>
> Thanks,
> Pankaj
>
>
--
Thanks,
David / dhildenb
- Re: [Qemu-devel] KVM "fake DAX" flushing interface - discussion, Pankaj Gupta, 2018/01/13
- Re: [Qemu-devel] KVM "fake DAX" flushing interface - discussion,
David Hildenbrand <=
- Re: [Qemu-devel] KVM "fake DAX" flushing interface - discussion, Dan Williams, 2018/01/18
- Re: [Qemu-devel] KVM "fake DAX" flushing interface - discussion, David Hildenbrand, 2018/01/18
- Re: [Qemu-devel] KVM "fake DAX" flushing interface - discussion, Dan Williams, 2018/01/18
- Re: [Qemu-devel] KVM "fake DAX" flushing interface - discussion, Pankaj Gupta, 2018/01/18
- Re: [Qemu-devel] KVM "fake DAX" flushing interface - discussion, Dan Williams, 2018/01/18
- Re: [Qemu-devel] KVM "fake DAX" flushing interface - discussion, Pankaj Gupta, 2018/01/18
- Re: [Qemu-devel] KVM "fake DAX" flushing interface - discussion, Dan Williams, 2018/01/18
- Re: [Qemu-devel] KVM "fake DAX" flushing interface - discussion, David Hildenbrand, 2018/01/18
- Re: [Qemu-devel] KVM "fake DAX" flushing interface - discussion, Dan Williams, 2018/01/18