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

[Dan Williams]

On Tue, May 30, 2017 at 5:17 AM, Paolo Bonzini <address@hidden> wrote:
>
>
> 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?
>

Yes, I believe it is similar. Device-dax guarantees and enforces (mmap
area must be aligned) a given fault granularity.