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

[David Gibson <david@gibson.dropbear.id.au>]

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On Thu, Apr 29, 2021 at 10:02:23PM +0530, Aneesh Kumar K.V wrote:
> On 4/29/21 9:25 PM, Stefan Hajnoczi wrote:
> > 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?
> > 
> 
> On virtualized ppc64 platforms, guests use papr_scm.ko kernel drive for
> persistent memory support. This was done such that we can use one kernel
> driver to support persistent memory with multiple hypervisors. To avoid
> supporting multiple drivers in the guest, -device nvdimm Qemu command-line
> results in Qemu using PAPR SCM backend. What this patch series does is to
> make sure we expose the correct synchronous fault support, when we back such
> nvdimm device with a file.
> 
> The existing PAPR SCM backend enables persistent memory support with the
> help of multiple hypercall.
> 
> #define H_SCM_READ_METADATA     0x3E4
> #define H_SCM_WRITE_METADATA    0x3E8
> #define H_SCM_BIND_MEM          0x3EC
> #define H_SCM_UNBIND_MEM        0x3F0
> #define H_SCM_UNBIND_ALL        0x3FC
> 
> Most of them are already implemented in Qemu. This patch series implements
> H_SCM_FLUSH hypercall.

The overall point here is that we didn't define the hypercall.  It was
defined in order to support NVDIMM/pmem devices under PowerVM.  For
uniformity between PowerVM and KVM guests, we want to support the same
hypercall interface on KVM/qemu as well.

-- 
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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