Re: [PATCH v4 0/3] nvdimm: Enable sync-dax property for nvdimm

Stefan Hajnoczi <[email protected]> Thu, 29 Apr 2021 16:55:13 +0100

Newsgroups	org.01.lists.linux-nvdimm,org.kernel.vger.kvm-ppc,org.nongnu.qemu-arm,org.nongnu.qemu-devel
Message-ID	<YIrW4bwbR1R0CWm/@stefanha-x1.localdomain>

On Wed, Apr 28, 2021 at 11:48:21PM -0400, Shivaprasad G Bhat wrote:
> The nvdimm devices are expected to ensure write persistence during power
> failure kind of scenarios.
> 
> The libpmem has architecture specific instructions like dcbf on POWER
> to flush the cache data to backend nvdimm device during normal writes
> followed by explicit flushes if the backend devices are not synchronous
> DAX capable.
> 
> Qemu - virtual nvdimm devices are memory mapped. The dcbf in the guest
> and the subsequent flush doesn't traslate to actual flush to the backend
> file on the host in case of file backed v-nvdimms. This is addressed by
> virtio-pmem in case of x86_64 by making explicit flushes translating to
> fsync at qemu.
> 
> On SPAPR, the issue is addressed by adding a new hcall to
> request for an explicit flush from the guest ndctl driver when the backend
> nvdimm cannot ensure write persistence with dcbf alone. So, the approach
> here is to convey when the hcall flush is required in a device tree
> property. The guest makes the hcall when the property is found, instead
> of relying on dcbf.

Sorry, I'm not very familiar with SPAPR. Why add a hypercall when the
virtio-nvdimm device already exists?

Stefan

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