Re: [PATCH v2] ArmVirtPkg: clear PcdPerformanceLibraryPropertyMask PCD

[Laszlo Ersek <lersek@redhat.com>]

On 02/27/17 16:09, Ard Biesheuvel wrote:
> The only observeable effect of having PcdPerformanceLibraryPropertyMask
> set to 1 is that a EfiReservedMemory region of 4 pages is allocated right
> below the 4 GB mark. This region is out of bounds for the OS, which means
> it is not even allowed to map it, to avoid speculative loads from it.
> 
> On Linux, this may prevent the kernel from using a 1 GB block mapping for
> this region, and instead it has to carve up the block as follows:
> 
>   0xffffffff80000000-0xffffffffbe000000         992M PMD CON BLK
>   0xffffffffbe000000-0xffffffffbfe00000          30M PMD     BLK
>   0xffffffffbfe00000-0xffffffffbfff0000        1984K PTE CON
>   0xffffffffbfff0000-0xffffffffbfffc000          48K PTE
> 
> where it would otherwise use a single 1 GB mapping (*), i.e.,
> 
>   0xffffffff80000000-0xffffffffc0000000           1G PGD
> 
> To clarify, the latter is a single 8 byte entry in the top level page
> table, whereas in the former case, we have two additional levels of
> paging, requiring two extra 4 KB pages (on a 4 KB pagesize kernel).
> 
> The real cost, however, is the TLB footprint, which goes up from a
> single entry to a number between 90 and 1020, depending on whether
> contiguous hints are honoured by the hardware.
> 
> So let's remove PcdPerformanceLibraryPropertyMask until we find a reason
> why we need it.
> 
> (*) provided that no other allocations were deliberately located right
>     below the 4 GB mark, and that we are running with more than 3 GB of
>     memory, in which case most allocations will be over 4 GB, given EDK2's
>     default top-down allocation policy.
> 
> Contributed-under: TianoCore Contribution Agreement 1.0
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
> ---
> v2: remove PCD altogether rather than explicitly set it to its default
>     value of 0
> 
>  ArmVirtPkg/ArmVirt.dsc.inc | 1 -
>  1 file changed, 1 deletion(-)
> 
> diff --git a/ArmVirtPkg/ArmVirt.dsc.inc b/ArmVirtPkg/ArmVirt.dsc.inc
> index 8e3d298723f1..cc09d38910a2 100644
> --- a/ArmVirtPkg/ArmVirt.dsc.inc
> +++ b/ArmVirtPkg/ArmVirt.dsc.inc
> @@ -295,7 +295,6 @@ [PcdsFixedAtBuild.common]
>    gEfiMdePkgTokenSpaceGuid.PcdMaximumLinkedListLength|1000000
>    gEfiMdePkgTokenSpaceGuid.PcdSpinLockTimeout|10000000
>    gEfiMdePkgTokenSpaceGuid.PcdDebugClearMemoryValue|0xAF
> -  gEfiMdePkgTokenSpaceGuid.PcdPerformanceLibraryPropertyMask|1
>    gEfiMdePkgTokenSpaceGuid.PcdPostCodePropertyMask|0
>    gEfiMdePkgTokenSpaceGuid.PcdUefiLibMaxPrintBufferSize|320
>  
> 

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