Re: [Qemu-devel] [RESEND PATCH 2/2] hostmem-file: add an attribute 'align' to set its alignment

[Paolo Bonzini]

On 26/05/2017 16:24, Dan Williams wrote:
>> For DAX device only, QEMU can figure out the proper alignment by
>> itself. However, I'm not sure whether there are other non-DAX cases
>> requiring non-default alignment, so I think it's better to just add an
>> interface (i.e. align attribute) in QEMU and let other management
>> tools (e.g. libvirt?) fill a proper value.
> I can't imagine any cases where you would want to specify an
> alignment. If it's regular file mmap any alignment is fine, and if
> it's device-dax only the configured alignment of the device instance
> is allowed. So, I don't think this should be a configurable option,
> just read it from the device instance and you're done.

A 2M or 1G alignment lets KVM use EPT hugepages if the host physical
addresses are contiguous and 2M- or 1G-aligned.

QEMU only does this for hugetlbfs currently, where the requirement on
the host physical addresses is always satisfied.  Would the same apply
to NVDIMM device DAX?

Thanks,

Paolo