Re: [Qemu-devel] [qemu PATCH v2 3/4] nvdimm, acpi: support NFIT platform capabilities

[Elliott, Robert (Persistent Memory)]

> -----Original Message-----
> From: Linux-nvdimm [mailto:address@hidden On Behalf Of
> Ross Zwisler
> Sent: Thursday, May 17, 2018 12:00 AM
> Subject: [qemu PATCH v2 3/4] nvdimm, acpi: support NFIT platform
> capabilities
> 
> Add a machine command line option to allow the user to control the
> Platform
> Capabilities Structure in the virtualized NFIT.  This Platform
> Capabilities
> Structure was added in ACPI 6.2 Errata A.
> 
...
> +Platform Capabilities
> +---------------------
> +
> +ACPI 6.2 Errata A added support for a new Platform Capabilities Structure
> +which allows the platform to communicate what features it supports
> related to
> +NVDIMM data durability.  Users can provide a capabilities value to a
> guest via
> +the optional "nvdimm-cap" machine command line option:
> +
> +    -machine pc,accel=kvm,nvdimm,nvdimm-cap=2
> +
> +As of ACPI 6.2 Errata A, the following values are valid for the bottom
> two
> +bits:
> +
> +2 - Memory Controller Flush to NVDIMM Durability on Power Loss Capable.
> +3 - CPU Cache Flush to NVDIMM Durability on Power Loss Capable.

It's a bit unclear that those are decimal values for the field, not 
bit numbers.

...
> -static GArray *nvdimm_build_device_structure(void)
> +/*
> + * ACPI 6.2 Errata A: 5.2.25.9 NVDIMM Platform Capabilities Structure
> + */
> +static void
> +nvdimm_build_structure_caps(GArray *structures, uint32_t capabilities)
> +{
> +    NvdimmNfitPlatformCaps *nfit_caps;
> +
> +    nfit_caps = acpi_data_push(structures, sizeof(*nfit_caps));
> +
> +    nfit_caps->type = cpu_to_le16(7 /* NVDIMM Platform Capabilities */);
> +    nfit_caps->length = cpu_to_le16(sizeof(*nfit_caps));
> +    nfit_caps->highest_cap = 2;
> +    nfit_caps->capabilities = cpu_to_le32(capabilities);

highest_cap needs to be set to a value that at least covers the highest 
bit set to 1 used in capabilities.

As capabilities bits are added, there are three different meanings:
* 1: bit within highest_cap range, platform is claiming the 1 meaning
* 0: bit within highest_cap range, platform is claiming the 0 meaning
* not reported: bit not within highest_cap range, so the platform's
  implementation of this feature is unknown. Not necessarily the same 
  as the 0 meaning.

So, there should be a way to specify a highest_cap value to convey that
some of the upper capabilities bits are valid and contain 0.

---
Robert Elliott, HPE Persistent Memory