Re: [Xen-devel] [PATCH for-4.5] tools/libxc: Avoid cacheflush toolstack hypercalls on x86

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Wed, 2014-09-24 at 14:47 +0100, Andrew Cooper wrote:
> XEN_DOMCTL_cacheflush hypercalls are (and will always be) -ENOSYS on x86, but
> xc_domain_cacheflush() is called often during domain build and migrate for
> correct behaviour on ARM.
> 
> Stub xc_domain_cacheflush() out on x86 to remove its pressure on the global
> domctl lock, and the hypercall overhead (which applies further pressure to the
> already heavily-contended TLB flush lock).
> 
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> CC: Keir Fraser <keir@xxxxxxx>
> CC: Jan Beulich <JBeulich@xxxxxxxx>
> CC: Tim Deegan <tim@xxxxxxx>
> CC: Ian Campbell <Ian.Campbell@xxxxxxxxxx>
> CC: Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>
> CC: Wei Liu <wei.liu2@xxxxxxxxxx>

Acked-by: Ian Campbell <ian.campbell@xxxxxxxxxx>

Although IMHO only the first paragraph of the comment is useful, the
rest belongs in the commit log (where it already mostly is already).

> ---
>  tools/libxc/xc_domain.c |   16 ++++++++++++++++
>  1 file changed, 16 insertions(+)
> 
> diff --git a/tools/libxc/xc_domain.c b/tools/libxc/xc_domain.c
> index 1a6f90a..a68127f 100644
> --- a/tools/libxc/xc_domain.c
> +++ b/tools/libxc/xc_domain.c
> @@ -51,12 +51,28 @@ int xc_domain_create(xc_interface *xch,
>  int xc_domain_cacheflush(xc_interface *xch, uint32_t domid,
>                           xen_pfn_t start_pfn, xen_pfn_t nr_pfns)
>  {
> +#if defined (__i386__) || defined (__x86_64__)
> +    /*
> +     * The x86 architecture provides cache coherency guarantees which prevent
> +     * the need for this hypercall.
> +     *
> +     * This call however is made frequently during domain build and migrate
> +     * for the benefit of ARM, where cache flushes are needed.
> +     *
> +     * Stub it out here on x86 as domctls take the global domctl lock in Xen
> +     * which is detrimental to concurrent toolstack operations on other
> +     * domains.
> +     */
> +    errno = ENOSYS;
> +    return -1;
> +#else
>      DECLARE_DOMCTL;
>      domctl.cmd = XEN_DOMCTL_cacheflush;
>      domctl.domain = (domid_t)domid;
>      domctl.u.cacheflush.start_pfn = start_pfn;
>      domctl.u.cacheflush.nr_pfns = nr_pfns;
>      return do_domctl(xch, &domctl);
> +#endif
>  }
>  
>  int xc_domain_pause(xc_interface *xch,



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