Re: [Xen-devel] [PATCH v2 11/15] xen/arm: Document the errata implemented in Xen

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Mon, 23 May 2016, Julien Grall wrote:
> The new document will help to keep track of all the erratum that Xen is
> able to handle.
> 
> The text is based on the Linux doc in Documents/arm64/silicon-errata.txt.
> 
> Also list the current errata that Xen is aware of.
> 
> Signed-off-by: Julien Grall <julien.grall@xxxxxxx>
> ---
>  docs/misc/arm/silicon-errata.txt | 45 
> ++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 45 insertions(+)
>  create mode 100644 docs/misc/arm/silicon-errata.txt
> 
> diff --git a/docs/misc/arm/silicon-errata.txt 
> b/docs/misc/arm/silicon-errata.txt
> new file mode 100644
> index 0000000..3f0d32b
> --- /dev/null
> +++ b/docs/misc/arm/silicon-errata.txt
> @@ -0,0 +1,45 @@
> +                Silicon Errata and Software Workarounds
> +                =======================================
> +
> +It is an unfortunate fact of life that hardware is often produced with
> +so-called "errata", which can cause it to deviate from the architecture
> +under specific circumstances.  For hardware produced by ARM, these
> +errata are broadly classified into the following categories:
> +
> +  Category A: A critical error without a viable workaround.
> +  Category B: A significant or critical error with an acceptable
> +              workaround.
> +  Category C: A minor error that is not expected to occur under normal
> +              operation.
> +
> +For more information, consult one of the "Software Developers Errata
> +Notice" documents available on infocenter.arm.com (registration
> +required).
> +
> +As far as Xen is concerned, Category B errata may require some special
> +treatment in the hypervisor. For example, avoiding a particular sequence
> +of code, or configuring the processor in a particular way. A less common
> +situation may require similar actions in order to declassify a Category A
> +erratum into a Category C erratum. These are collectively known as
> +"software workarounds" and are only required in the minority of cases
> +(e.g. those cases that both require a non-secure workaround *and* can
> +be triggered by Linux).
                   ^by Xen


> +For software workarounds that may adversely impact systems unaffected by
> +the erratum in question, a Kconfig entry is added under "ARM errata
> +workarounds via the alternatives framework". These are enabled by default
> +and patched in at runtime when an affected CPU is detected. For
> +less-intrusive workarounds, a Kconfig option is not available and the code
> +is structured (preferably with a comment) in such a way that the erratum
> +will not be hit.
> +
> +This approach can make it slightly onerous to determine exactly which
> +errata are worked around in an arbitrary hypervisor source tree, so this
> +file acts as a registry of software workarounds in the Xen hypervisor and
> +will be updated when new workarounds are committed and backported to
> +stable hypervisors.
> +
> +| Implementor    | Component       | Erratum ID      | Kconfig               
>   |
> ++----------------+-----------------+-----------------+-------------------------+
> +| ARM            | Cortex-A15      | #766422         | N/A                   
>   |
> +| ARM            | Cortex-A57      | #852523         | N/A                   
>   |
> -- 
> 1.9.1
> 

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