Re: [Xen-devel] [PATCH] x86/libxl: choose a sane default for HAP

[Jan Beulich <jbeulich@xxxxxxxx>]

On 05.09.2019 11:34, Roger Pau Monne wrote:
> Current libxl code will always enable Hardware Assisted Paging (HAP),
> expecting that the hypervisor will fallback to shadow if HAP is not
> available. With the changes to the domain builder that's not the case
> any longer, and the hypervisor will raise an error if HAP is not
> available instead of silently falling back to shadow.

Would it really be much more involved than the change here to restore
silent defaulting to shadow?

> --- a/xen/include/public/sysctl.h
> +++ b/xen/include/public/sysctl.h
> @@ -90,6 +90,10 @@ struct xen_sysctl_tbuf_op {
>   /* (x86) The platform supports direct access to I/O devices with IOMMU. */
>  #define _XEN_SYSCTL_PHYSCAP_directio     2
>  #define XEN_SYSCTL_PHYSCAP_directio  (1u<<_XEN_SYSCTL_PHYSCAP_directio)
> +/* (x86) The platform supports Hardware Assisted Paging. */
> +#define _XEN_SYSCTL_PHYSCAP_hap          3
> +#define XEN_SYSCTL_PHYSCAP_hap           (1u<<_XEN_SYSCTL_PHYSCAP_hap)

I think this bit wants to be universal (i.e. "(x86)" dropped), and
be set unconditionally on Arm. Irrespective of the question regarding
an alternative solution I think it is quite sensible to expose
availability of HAP to the tools. In fact I think "xl info" should
show this alongside other properties.

Jan

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