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Subject: Re: [PATCH v3 27/35] OvmfPkg/XenPlatformLib: Cache result for XenDetected
To: Anthony PERARD <anthony.perard@citrix.com>, devel@edk2.groups.io
Cc: xen-devel@lists.xenproject.org, Ard Biesheuvel
 <ard.biesheuvel@linaro.org>, Jordan Justen <jordan.l.justen@intel.com>,
 Julien Grall <julien.grall@arm.com>
References: <20190704144233.27968-1-anthony.perard@citrix.com>
 <20190704144233.27968-28-anthony.perard@citrix.com>
From: "Laszlo Ersek" <lersek@redhat.com>
Message-ID: <f2c7ed3d-c453-1324-6d2d-d004154b6c9e@redhat.com>
Date: Wed, 10 Jul 2019 11:31:23 +0200
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On 07/04/19 16:42, Anthony PERARD wrote:
> We are going to replace XenDetected() implementation in
> PlatformBootManagerLib by the one in XenPlatformLib.
> PlatformBootManagerLib's implementation does cache the result of
> GetFirstGuidHob(), so we do something similar in XenPlatformLib.
> 
> Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=1689
> Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
> ---
> 
> Notes:
>     v3:
>     - new patch
> 
>  .../Library/XenPlatformLib/XenPlatformLib.c    | 18 +++++++++++++++---
>  1 file changed, 15 insertions(+), 3 deletions(-)
> 
> diff --git a/OvmfPkg/Library/XenPlatformLib/XenPlatformLib.c b/OvmfPkg/Library/XenPlatformLib/XenPlatformLib.c
> index 6f27cbffa8..b5257b0c97 100644
> --- a/OvmfPkg/Library/XenPlatformLib/XenPlatformLib.c
> +++ b/OvmfPkg/Library/XenPlatformLib/XenPlatformLib.c
> @@ -26,13 +26,25 @@ XenGetInfoHOB (
>    )
>  {
>    EFI_HOB_GUID_TYPE  *GuidHob;
> +  STATIC BOOLEAN     Cached = FALSE;
> +  STATIC EFI_XEN_INFO *XenInfo;

(1) The alignment of the variable names is weird. The above is neither
condensed nor precisely aligned. Please pick one:

  EFI_HOB_GUID_TYPE *GuidHob;
  STATIC BOOLEAN Cached = FALSE;
  STATIC EFI_XEN_INFO *XenInfo;

or

  EFI_HOB_GUID_TYPE   *GuidHob;
  STATIC BOOLEAN      Cached = FALSE;
  STATIC EFI_XEN_INFO *XenInfo;

(The 2nd form is preferred in edk2.)

> +
> +  //
> +  // Return the cached result for the benefit of XenDetected that can be
> +  // called many times.
> +  //
> +  if (Cached) {
> +    return XenInfo;
> +  }
>  
>    GuidHob = GetFirstGuidHob (&gEfiXenInfoGuid);
>    if (GuidHob == NULL) {
> -    return NULL;
> +    XenInfo = NULL;
> +  } else {
> +    XenInfo = (EFI_XEN_INFO *) GET_GUID_HOB_DATA (GuidHob);
>    }
> -
> -  return (EFI_XEN_INFO *) GET_GUID_HOB_DATA (GuidHob);
> +  Cached = TRUE;
> +  return XenInfo;
>  }
>  
>  /**
> 

This will work fine in DXE modules (and by the end of the series, only
DXE modules use XenPlatformLib -- AcpiPlatformDxe, XenIoPvhDxe, and
PlatformBootManagerLib, which is only linked into DXE modules).

With (1) fixed:

Reviewed-by: Laszlo Ersek <lersek@redhat.com>

Thanks
Laszlo