On Wed, Mar 24, 2021 at 09:34:06AM +0530, Aneesh Kumar K.V wrote: > On 3/24/21 8:37 AM, David Gibson wrote: > > On Tue, Mar 23, 2021 at 09:47:38AM -0400, Shivaprasad G Bhat wrote: > > > The patch adds support for the SCM flush hcall for the nvdimm devices. > > > To be available for exploitation by guest through the next patch. > > > > > > The hcall expects the semantics such that the flush to return > > > with H_BUSY when the operation is expected to take longer time along > > > with a continue_token. The hcall to be called again providing the > > > continue_token to get the status. So, all fresh requsts are put into > > > a 'pending' list and flush worker is submitted to the thread pool. > > > The thread pool completion callbacks move the requests to 'completed' > > > list, which are cleaned up after reporting to guest in subsequent > > > hcalls to get the status. > > > > > > The semantics makes it necessary to preserve the continue_tokens > > > and their return status even across migrations. So, the pre_save > > > handler for the device waits for the flush worker to complete and > > > collects all the hcall states from 'completed' list. The necessary > > > nvdimm flush specific vmstate structures are added to the spapr > > > machine vmstate. > > > > > > Signed-off-by: Shivaprasad G Bhat <sbhat@xxxxxxxxxxxxx> > > > > An overal question: surely the same issue must arise on x86 with > > file-backed NVDIMMs. How do they handle this case? > > On x86 we have different ways nvdimm can be discovered. ACPI NFIT, e820 map > and virtio_pmem. Among these virio_pmem always operated with synchronous dax > disabled and both ACPI and e820 doesn't have the ability to differentiate > support for synchronous dax. Ok. And for the virtio-pmem case, how are the extra flushes actually done on x86? > With that I would expect users to use virtio_pmem when using using file > backed NVDIMMS So... should we prevent advertising an NVDIMM through ACPI or e820 if it doesn't have sync-dax enabled? -- David Gibson | I'll have my music baroque, and my code david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_ | _way_ _around_! http://www.ozlabs.org/~dgibson
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