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Re: [Qemu-devel] [PATCH v2] configure: enable --s390-pgste linker option


From: Christian Borntraeger
Subject: Re: [Qemu-devel] [PATCH v2] configure: enable --s390-pgste linker option
Date: Wed, 23 Aug 2017 08:55:14 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.2.0


On 08/23/2017 08:53 AM, Christian Borntraeger wrote:
> KVM guests on s390 need a different page table layout than normal
> processes (2kb page table + 2kb page status extensions vs 2kb page table
> only). As of today this has to be enabled via the vm.allocate_pgste
> sysctl.
> 
> Newer kernels (>= 4.12) on s390 check for an S390_PGSTE program header
> and enable the pgste page table extensions in that case. This makes the
> vm.allocate_pgste sysctl unnecessary. We enable this program header for
> the s390 system emulation (qemu-system-s390x) if we build on s390
> - for s390 system emulation
> - the linker supports --s390-pgste (binutils >= 2.29)
> - KVM is enabled
> 
> This will allow distributions to disable the global vm.allocate_pgste
> sysctl, which will improve the page table allocation for non KVM
> processes as only 2kb chunks are necessary.
> 
> Cc: Christian Ehrhardt <address@hidden>
> Cc: Alexander Graf <address@hidden>
> Cc: Dan Horak <address@hidden>
> Cc: David Hildenbrand <address@hidden>
> Signed-off-by: Christian Borntraeger <address@hidden>
> Acked-by: Janosch Frank <address@hidden>
> ---
> V1->V2:
>       - provide ld_has function
>       - use ld_has to replace some open coded variants
>       - check target arch and arch for s390
>       - check for s390x before calling the linker
> 
>  configure | 21 ++++++++++++++++++++-
>  1 file changed, 20 insertions(+), 1 deletion(-)
> 
> diff --git a/configure b/configure
> index dd73cce..0b68b37 100755
> --- a/configure
> +++ b/configure
> @@ -240,6 +240,11 @@ supported_target() {
>      return 1
>  }
> 
> +
> +ld_has() {
> +    $ld --help 2>/dev/null | grep ".$1" >/dev/null 2>&1
> +}
> +
>  # default parameters
>  source_path=$(dirname "$0")
>  cpu=""
> @@ -5043,7 +5048,7 @@ fi
>  # Use ASLR, no-SEH and DEP if available
>  if test "$mingw32" = "yes" ; then
>      for flag in --dynamicbase --no-seh --nxcompat; do
> -        if $ld --help 2>/dev/null | grep ".$flag" >/dev/null 2>/dev/null ; 
> then
> +        if ld_had $flag ; then

        typo, will fix with v3 :-/


>              LDFLAGS="-Wl,$flag $LDFLAGS"
>          fi
>      done
> @@ -6522,6 +6527,20 @@ if test "$target_linux_user" = "yes" -o 
> "$target_bsd_user" = "yes" ; then
>    ldflags="$ldflags $textseg_ldflags"
>  fi
> 
> +# Newer kernels on s390 check for an S390_PGSTE program header and
> +# enable the pgste page table extensions in that case. This makes
> +# the vm.allocate_pgste sysctl unnecessary. We enable this program
> +# header if
> +#  - we build on s390x
> +#  - we build the system emulation for s390x (qemu-system-s390x)
> +#  - KVM is enabled
> +#  - the linker support --s390-pgste
> +if test "$TARGET_ARCH" = "s390x" -a "$target_softmmu" = "yes"  -a "$ARCH" = 
> "s390x" -a "$kvm" = "yes"; then
> +    if ld_has --s390-pgste ; then
> +        ldflags="-Wl,--s390-pgste $ldflags"
> +    fi
> +fi
> +
>  echo "LDFLAGS+=$ldflags" >> $config_target_mak
>  echo "QEMU_CFLAGS+=$cflags" >> $config_target_mak
> 




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