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


From: Paolo Bonzini
Subject: Re: [Qemu-devel] [PATCH v2] configure: enable --s390-pgste linker option
Date: Wed, 23 Aug 2017 10:16:27 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.2.1

On 23/08/2017 10:06, Thomas Huth wrote:
> On 23.08.2017 10:00, David Hildenbrand wrote:
>> On 23.08.2017 08:53, 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
>>>  }
>>>  
>>> +
>>
>> I'd drop this new line
>>
>>> +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 $flag ; then
>>>              LDFLAGS="-Wl,$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 "$kvm" = "yes"; then
>>
>> Wonder if the "$ARCH" check is really necessary: TARGET_ARCH=s390x with
>> kvm=yes should only build on s390x.
> 
> Isn't kvm=yes and TARGET_ARCH=s390x also possible on a x86 host, where
> only the x86_64 target is built with CONFIG_KVM=y, but the s390x target
> with CONFIG_KVM=n ?

Yes.  You could use

  if test "$ARCH" = "s390x" && supported_kvm_target $target; then
    ...
  fi

Or, in the existing "if supported_kvm_target $target" conditional, add

  if test "$ARCH" = s390x && ld_has --s390-pgste; then
    ...
  fi

Paolo



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