Re: [Qemu-devel] [RESEND PATCH 1/2] nvdimm: warn if the backend is not a DAX device

[Dan Williams]

On Thu, May 25, 2017 at 9:30 PM, Haozhong Zhang
<address@hidden> wrote:
> On 05/25/17 20:34 -0700, Dan Williams wrote:
>> On Thu, May 25, 2017 at 7:32 PM, Haozhong Zhang
>> <address@hidden> wrote:
>> > Applications in Linux guest that use device-dax never trigger flush
>> > that can be trapped by KVM/QEMU. Meanwhile, if the host backend is not
>> > device-dax, QEMU cannot guarantee the persistence of guest writes.
>> > Before solving this flushing problem, QEMU should warn users if the
>> > host backend is not device-dax.
>>
>> I think this needs to be stronger than a "warn" it needs to be
>> explicitly forbidden when it is known to be unsafe.
>>
>
> I understand your worry and am not object to your suggestion, but
> forbidden will change the existing behavior that allows such unsafe
> usage. Let's wait for other maintainers' comments.

Changing existing behavior is the point. Even if a qemu maintainer
thought it was acceptable to allow the dangerous behavior with a
warning I would continue to assert that we need to block it by
default. I'm not opposed to adding a new configuration option like
"allow dangerous pmem" to override the new default behavior, but I
think this patch is incorrect to just emit a message.