Re: [PATCH RESEND v4] nvme: introduce PMR support from NVMe 1.4 spec

[Andrzej Jakowski]

On 3/30/20 12:31 PM, Klaus Birkelund Jensen wrote:
> On Mar 31 03:13, Keith Busch wrote:
>> On Mon, Mar 30, 2020 at 08:07:32PM +0200, Klaus Birkelund Jensen wrote:
>>> On Mar 31 01:55, Keith Busch wrote:
>>>> On Mon, Mar 30, 2020 at 09:46:56AM -0700, Andrzej Jakowski wrote:
>>>>> This patch introduces support for PMR that has been defined as part of 
>>>>> NVMe 1.4
>>>>> spec. User can now specify a pmrdev option that should point to 
>>>>> HostMemoryBackend.
>>>>> pmrdev memory region will subsequently be exposed as PCI BAR 2 in 
>>>>> emulated NVMe
>>>>> device. Guest OS can perform mmio read and writes to the PMR region that 
>>>>> will stay
>>>>> persistent across system reboot.
>>>>
>>>> Looks pretty good to me.
>>>>
>>>> Reviewed-by: Keith Busch <address@hidden>
>>>>
>>>> For a possible future extention, it could be interesting to select the
>>>> BAR for PMR dynamically, so that you could have CMB and PMR enabled on
>>>> the same device.
>>>>
>>>
>>> I thought about the same thing, but this would require disabling MSI-X
>>> right? Otherwise there is not enough room. AFAIU, the iomem takes up
>>> BAR0 and BAR1, CMB (or PMR) uses BAR2 and BAR3 and MSI-X uses BAR4 or 5.
>>
>> That's the way the emulated device is currently set, but there's no
>> reason for CMB or MSIx to use an exlusive bar. Both of those can be
>> be offsets into BAR 0/1, for example. PMR is the only one that can't
>> share.
>>
> 
> Oh, I did not consider that at all. Thanks!
> 
Makes sense - I didn't realize that it is possible. Thx Keith! 
Let me propose to introduce it in a separate patch.