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

[Stefan Hajnoczi]

On Fri, Apr 30, 2021 at 02:27:18PM +1000, David Gibson wrote:
> 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.

Okay, that's fine. Now Linux and QEMU have multiple ways of doing this,
but it's fair enough if it's an existing platform hypercall.

Stefan

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