Re: KVM "fake DAX" flushing interface - discussion

[Xiao Guangrong <xiaoguangrong.eric@gmail.com>]

On 10/31/2017 10:20 PM, Dan Williams wrote:
> On Tue, Oct 31, 2017 at 12:13 AM, Xiao Guangrong
> <xiaoguangrong.eric@gmail.com> wrote:
>>
>>
>> On 07/27/2017 08:54 AM, Dan Williams wrote:
>>
>>>> At that point, would it make sense to expose these special
>>>> virtio-pmem areas to the guest in a slightly different way,
>>>> so the regions that need virtio flushing are not bound by
>>>> the regular driver, and the regular driver can continue to
>>>> work for memory regions that are backed by actual pmem in
>>>> the host?
>>>
>>>
>>> Hmm, yes that could be feasible especially if it uses the ACPI NFIT
>>> mechanism. It would basically involve defining a new SPA (System
>>> Phyiscal Address) range GUID type, and then teaching libnvdimm to
>>> treat that as a new pmem device type.
>>
>>
>> I would prefer a new flush mechanism to a new memory type introduced
>> to NFIT, e.g, in that mechanism we can define request queues and
>> completion queues and any other features to make virtualization
>> friendly. That would be much simpler.
>>
> 
> No that's more confusing because now we are overloading the definition
> of persistent memory. I want this memory type identified from the top
> of the stack so it can appear differently in /proc/iomem and also
> implement this alternate flush communication.
> 

For the characteristic of memory, I have no idea why VM should know this
difference. It can be completely transparent to VM, that means, VM
does not need to know where this virtual PMEM comes from (for a really
nvdimm backend or a normal storage). The only discrepancy is the flush
interface.

> In what way is this "more complicated"? It was trivial to add support
> for the "volatile" NFIT range, this will not be any more complicated
> than that.
> 

Introducing memory type is easy indeed, however, a new flush interface
definition is inevitable, i.e, we need a standard way to discover the
MMIOs to communicate with host.
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