Re: [PATCH v2 09/10] OvmfPkg: disable list length checks in NOOPT and DEBUG builds

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

On 1/13/21 9:54 AM, Laszlo Ersek wrote:
> In NOOPT and DEBUG builds, if "PcdMaximumLinkedListLength" is nonzero,
> then several LIST_ENTRY *node* APIs in BaseLib compare the *full* list
> length against the PCD.
> 
> This turns the time complexity of node-level APIs from constant to linear,
> and that of full-list manipulations from linear to quadratic.
> 
> As an example, consider the EFI_SHELL_FILE_INFO list, which is a data
> structure that's widely used in the UEFI shell. I randomly extracted 5000
> files from "/usr/include" on my laptop, spanning 1095 subdirectories out
> of 1538, and then ran "DIR -R" in the UEFI shell on this tree. These are
> the wall-clock times:
> 
>            PcdMaximumLinkedListLength  PcdMaximumLinkedListLength
>            =1,000,000                  =0
>            --------------------------  ---------------------------
> FAT        4 min 31 s                        18 s
> virtio-fs  5 min 13 s                  1 min 33 s
> 
> Checking list lengths against an arbitrary maximum (default: 1,000,000)
> seems useless even in NOOPT and DEBUG builds, while the cost is
> significant; so set the PCD to 0.
> 
> Cc: Anthony Perard <anthony.perard@citrix.com>
> Cc: Ard Biesheuvel <ard.biesheuvel@arm.com>
> Cc: Jordan Justen <jordan.l.justen@intel.com>
> Cc: Julien Grall <julien@xen.org>
> Cc: Leif Lindholm <leif@nuviainc.com>
> Cc: Peter Grehan <grehan@freebsd.org>
> Cc: Philippe Mathieu-Daudé <philmd@redhat.com>
> Cc: Rebecca Cran <rebecca@bsdio.com>
> Cc: Sami Mujawar <sami.mujawar@arm.com>
> Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=3152
> Signed-off-by: Laszlo Ersek <lersek@redhat.com>
> Acked-by: Ard Biesheuvel <ard.biesheuvel@arm.com>
> ---
> 
> Notes:
>     v2:
>     - no changes
>     - pick up Ard's A-b
> 
>  OvmfPkg/AmdSev/AmdSevX64.dsc | 1 +
>  OvmfPkg/Bhyve/BhyveX64.dsc   | 1 +
>  OvmfPkg/OvmfPkgIa32.dsc      | 1 +
>  OvmfPkg/OvmfPkgIa32X64.dsc   | 1 +
>  OvmfPkg/OvmfPkgX64.dsc       | 1 +
>  OvmfPkg/OvmfXen.dsc          | 1 +
>  6 files changed, 6 insertions(+)

Reviewed-by: Philippe Mathieu-Daude <philmd@redhat.com>