Re: [Xen-devel] [PATCH v5 12/12] x86/hyperv: setup VP assist page

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Wed, Jan 29, 2020 at 08:20:34PM +0000, Wei Liu wrote:
> VP assist page is rather important as we need to toggle some bits in it
> for efficient nested virtualisation.
> 
> Signed-off-by: Wei Liu <liuwe@xxxxxxxxxxxxx>
> ---
> v5:
> 1. Deal with error properly instead of always panicking
> 2. Swap percpu variables declarations' location
> 
> v4:
> 1. Use private.h
> 2. Prevent leak
> 
> v3:
> 1. Use xenheap page
> 2. Drop set_vp_assist
> 
> v2:
> 1. Use HV_HYP_PAGE_SHIFT instead
> ---
>  xen/arch/x86/guest/hyperv/hyperv.c  | 44 ++++++++++++++++++++++++++++-
>  xen/arch/x86/guest/hyperv/private.h |  1 +
>  2 files changed, 44 insertions(+), 1 deletion(-)
> 
> diff --git a/xen/arch/x86/guest/hyperv/hyperv.c 
> b/xen/arch/x86/guest/hyperv/hyperv.c
> index af0d6ed692..bc40a3d338 100644
> --- a/xen/arch/x86/guest/hyperv/hyperv.c
> +++ b/xen/arch/x86/guest/hyperv/hyperv.c
> @@ -31,6 +31,7 @@
>  
>  struct ms_hyperv_info __read_mostly ms_hyperv;
>  DEFINE_PER_CPU_READ_MOSTLY(void *, hv_pcpu_input_page);
> +DEFINE_PER_CPU_READ_MOSTLY(void *, hv_vp_assist);
>  DEFINE_PER_CPU_READ_MOSTLY(unsigned int, hv_vp_index);
>  
>  static uint64_t generate_guest_id(void)
> @@ -155,16 +156,57 @@ static int setup_hypercall_pcpu_arg(void)
>      return 0;
>  }
>  
> +static int setup_vp_assist(void)
> +{
> +    void *mapping;
> +    uint64_t val;
> +
> +    mapping = this_cpu(hv_vp_assist);

You could also avoid the usage of the local mapping variable here.

> +
> +    if ( !mapping )
> +    {
> +        mapping = alloc_xenheap_page();
> +        if ( !mapping )
> +        {
> +            printk("Failed to allocate vp_assist page for CPU%u\n",
> +                   smp_processor_id());
> +            return -ENOMEM;
> +        }
> +
> +        clear_page(mapping);
> +        this_cpu(hv_vp_assist) = mapping;
> +    }
> +
> +    val = (virt_to_mfn(mapping) << HV_HYP_PAGE_SHIFT)

There's virt_to_maddr which would avoid the shift.

> +        | HV_X64_MSR_VP_ASSIST_PAGE_ENABLE;
> +    wrmsrl(HV_X64_MSR_VP_ASSIST_PAGE, val);
> +
> +    return 0;
> +}
> +
>  static void __init setup(void)
>  {
>      setup_hypercall_page();
> +
>      if ( setup_hypercall_pcpu_arg() )
>          panic("Hypercall percpu arg setup failed\n");
> +
> +    if ( setup_vp_assist() )
> +        panic("VP assist page setup failed\n");
>  }
>  
>  static int ap_setup(void)
>  {
> -    return setup_hypercall_pcpu_arg();
> +    int rc;
> +
> +    rc = setup_hypercall_pcpu_arg();
> +    if ( rc )
> +        goto out;

No need for a label, as just returning here would make the function
shorter:

rc = setup_hypercall_pcpu_arg();
if ( rc )
    return rc;

return setup_vp_assist();

Thanks, Roger.

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