Re: [PATCH v3 2/2] x86/hvm/viridian: Enable APIC assist enlightenment

[Wei Liu <wei.liu2@citrix.com>]

On Wed, Mar 16, 2016 at 05:44:12PM +0000, Paul Durrant wrote:
> This patch adds code to enable the APIC assist enlightenment which,
> under certain conditions, means that the guest can avoid an EOI of
> the local APIC and thereby avoid a VMEXIT.
> 
> Use of the enlightenment by the hypervisor is under control of the
> toolstack, and is added to the default set.
> 
> Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
> Cc: Ian Jackson <ian.jackson@eu.citrix.com>
> Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
> Cc: Wei Liu <wei.liu2@citrix.com>
> Cc: Keir Fraser <keir@xen.org>
> Cc: Jan Beulich <jbeulich@suse.com>
> Cc: Andrew Cooper <andrew.cooper3@citrix.com>
> ---
> 
> v3:
>  - Re-instated read-modify-write for forwards compatibility
>  - Fix a coding style issue
> 
> v2:
>  - Removed some code duplication and unnecessary read-modify-write
>    operations on the APIC assist word.
>  - Stated in the xl.cfg text that the enlightenment has no effect if
>    posted interrupts are in use.
>  - Added the enlightenment to the default set.
> ---
>  docs/man/xl.cfg.pod.5              | 13 ++++++++-
>  tools/libxl/libxl_dom.c            |  4 +++
>  tools/libxl/libxl_types.idl        |  1 +
>  xen/arch/x86/hvm/viridian.c        | 59 ++++++++++++++++++++++++++++++++++----
>  xen/arch/x86/hvm/vlapic.c          | 58 +++++++++++++++++++++++++++++++++----
>  xen/include/asm-x86/hvm/viridian.h |  5 ++++
>  xen/include/public/hvm/params.h    |  7 ++++-
>  7 files changed, 134 insertions(+), 13 deletions(-)
> 
> diff --git a/docs/man/xl.cfg.pod.5 b/docs/man/xl.cfg.pod.5
> index 56b1117..1ba026b 100644
> --- a/docs/man/xl.cfg.pod.5
> +++ b/docs/man/xl.cfg.pod.5
> @@ -1484,10 +1484,21 @@ This set incorporates use of hypercalls for remote TLB flushing.
>  This enlightenment may improve performance of Windows guests running
>  on hosts with higher levels of (physical) CPU contention.
>  
> +=item B<apic_assist>
> +
> +This set incorporates use of the APIC assist page to avoid EOI of
> +the local APIC.
> +This enlightenment may improve performance of Windows guests,
> +particularly those running PV drivers that make use of per-vcpu
> +event channel upcall vectors.

I think this can be changed to

  This enlightenment may improve performance of guests that make use of
  per-vcpu event channel upcall vectors.

Surely Linux can benefit from this too, right?

> +Note that this enlightenment will have no effect if the guest is
> +using APICv posted interrupts.
> +
>  =item B<defaults>
>  
>  This is a special value that enables the default set of groups, which
> -is currently the B<base>, B<freq> and B<time_ref_count> groups.
> +is currently the B<base>, B<freq>, B<time_ref_count> and B<apic_assist>
> +groups.
>  
>  =item B<all>
>  
> diff --git a/tools/libxl/libxl_dom.c b/tools/libxl/libxl_dom.c
> index b825b98..09d3bca 100644
> --- a/tools/libxl/libxl_dom.c
> +++ b/tools/libxl/libxl_dom.c
> @@ -211,6 +211,7 @@ static int hvm_set_viridian_features(libxl__gc *gc, uint32_t domid,
>          libxl_bitmap_set(&enlightenments, LIBXL_VIRIDIAN_ENLIGHTENMENT_BASE);
>          libxl_bitmap_set(&enlightenments, LIBXL_VIRIDIAN_ENLIGHTENMENT_FREQ);
>          libxl_bitmap_set(&enlightenments, LIBXL_VIRIDIAN_ENLIGHTENMENT_TIME_REF_COUNT);
> +        libxl_bitmap_set(&enlightenments, LIBXL_VIRIDIAN_ENLIGHTENMENT_APIC_ASSIST);
>      }
>  
>      libxl_for_each_set_bit(v, info->u.hvm.viridian_enable) {
> @@ -253,6 +254,9 @@ static int hvm_set_viridian_features(libxl__gc *gc, uint32_t domid,
>      if (libxl_bitmap_test(&enlightenments, LIBXL_VIRIDIAN_ENLIGHTENMENT_HCALL_REMOTE_TLB_FLUSH))
>          mask |= HVMPV_hcall_remote_tlb_flush;
>  
> +    if (libxl_bitmap_test(&enlightenments, LIBXL_VIRIDIAN_ENLIGHTENMENT_APIC_ASSIST))
> +        mask |= HVMPV_apic_assist;
> +
>      if (mask != 0 &&
>          xc_hvm_param_set(CTX->xch,
>                           domid,
> diff --git a/tools/libxl/libxl_types.idl b/tools/libxl/libxl_types.idl
> index 632c009..e3be957 100644
> --- a/tools/libxl/libxl_types.idl
> +++ b/tools/libxl/libxl_types.idl
> @@ -221,6 +221,7 @@ libxl_viridian_enlightenment = Enumeration("viridian_enlightenment", [
>      (2, "time_ref_count"),
>      (3, "reference_tsc"),
>      (4, "hcall_remote_tlb_flush"),
> +    (5, "apic_assist"),
>      ])
>  


Missing a LIBXL_HAVE macro in libxl.h for this new field.

Wei.

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