diff mbox series

[11/26] deprecation: don't enable TCG plugins by default on 32 bit hosts

Message ID 20240910140733.4007719-12-alex.bennee@linaro.org
State New
Headers show
Series Maintainer updates (testing, gdbstub, plugins) | expand

Commit Message

Alex Bennée Sept. 10, 2024, 2:07 p.m. UTC
The existing plugins already liberally use host pointer stuffing for
passing user data which will fail when doing 64 bit guests on 32 bit
hosts. We should discourage this by officially deprecating support and
adding another nail to the 32 bit host coffin.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>

---
v2
  - don't manually set based on CPU type, use __SIZEOF_POINTER__
---
 docs/about/deprecated.rst | 11 +++++++++++
 configure                 | 21 ++++++++++++++++++++-
 2 files changed, 31 insertions(+), 1 deletion(-)

Comments

Pierrick Bouvier Sept. 10, 2024, 2:45 p.m. UTC | #1
On 9/10/24 07:07, Alex Bennée wrote:
> The existing plugins already liberally use host pointer stuffing for
> passing user data which will fail when doing 64 bit guests on 32 bit
> hosts. We should discourage this by officially deprecating support and
> adding another nail to the 32 bit host coffin.
> 
> Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
> 
> ---
> v2
>    - don't manually set based on CPU type, use __SIZEOF_POINTER__
> ---
>   docs/about/deprecated.rst | 11 +++++++++++
>   configure                 | 21 ++++++++++++++++++++-
>   2 files changed, 31 insertions(+), 1 deletion(-)
> 
> diff --git a/docs/about/deprecated.rst b/docs/about/deprecated.rst
> index 88f0f03786..f7c7c33d39 100644
> --- a/docs/about/deprecated.rst
> +++ b/docs/about/deprecated.rst
> @@ -184,6 +184,17 @@ be an effective use of its limited resources, and thus intends to discontinue
>   it. Since all recent x86 hardware from the past >10 years is capable of the
>   64-bit x86 extensions, a corresponding 64-bit OS should be used instead.
>   
> +TCG Plugin support not enabled by default on 32-bit hosts (since 9.2)
> +'''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''
> +
> +While it is still possible to enable TCG plugin support for 32-bit
> +hosts there are a number of potential pitfalls when instrumenting
> +64-bit guests. The plugin APIs typically pass most addresses as
> +uint64_t but practices like encoding that address in a host pointer
> +for passing as user-data will lose data. As most software analysis
> +benefits from having plenty of host memory it seems reasonable to
> +encourage users to use 64 bit builds of QEMU for analysis work
> +whatever targets they are instrumenting.
>   
>   System emulator CPUs
>   --------------------
> diff --git a/configure b/configure
> index 40186d865e..14581c1b9a 100755
> --- a/configure
> +++ b/configure
> @@ -516,6 +516,25 @@ case "$cpu" in
>       ;;
>   esac
>   
> +# Now we have our CPU_CFLAGS we can check if we are targeting a 32 or
> +# 64 bit host.
> +
> +check_64bit_host() {
> +cat > $TMPC <<EOF
> +#if __SIZEOF_POINTER__ != 8
> +#error not 64 bit system
> +#endif
> +int main(void) { return 0; }
> +EOF
> +  compile_object "$1"
> +}
> +
> +if check_64bit_host "$CPU_CFLAGS"; then
> +    host_bits=64
> +else
> +    host_bits=32
> +fi
> +
>   if test -n "$host_arch" && {
>       ! test -d "$source_path/linux-user/include/host/$host_arch" ||
>       ! test -d "$source_path/common-user/host/$host_arch"; }; then
> @@ -1028,7 +1047,7 @@ if test "$static" = "yes" ; then
>     fi
>     plugins="no"
>   fi
> -if test "$plugins" != "no"; then
> +if test "$plugins" != "no" && test $host_bits -eq 64; then
>     plugins=yes
>     subdirs="$subdirs contrib/plugins"
>   fi

Thanks, it should open the door to be able to build plugins with meson.
Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
diff mbox series

Patch

diff --git a/docs/about/deprecated.rst b/docs/about/deprecated.rst
index 88f0f03786..f7c7c33d39 100644
--- a/docs/about/deprecated.rst
+++ b/docs/about/deprecated.rst
@@ -184,6 +184,17 @@  be an effective use of its limited resources, and thus intends to discontinue
 it. Since all recent x86 hardware from the past >10 years is capable of the
 64-bit x86 extensions, a corresponding 64-bit OS should be used instead.
 
+TCG Plugin support not enabled by default on 32-bit hosts (since 9.2)
+'''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''
+
+While it is still possible to enable TCG plugin support for 32-bit
+hosts there are a number of potential pitfalls when instrumenting
+64-bit guests. The plugin APIs typically pass most addresses as
+uint64_t but practices like encoding that address in a host pointer
+for passing as user-data will lose data. As most software analysis
+benefits from having plenty of host memory it seems reasonable to
+encourage users to use 64 bit builds of QEMU for analysis work
+whatever targets they are instrumenting.
 
 System emulator CPUs
 --------------------
diff --git a/configure b/configure
index 40186d865e..14581c1b9a 100755
--- a/configure
+++ b/configure
@@ -516,6 +516,25 @@  case "$cpu" in
     ;;
 esac
 
+# Now we have our CPU_CFLAGS we can check if we are targeting a 32 or
+# 64 bit host.
+
+check_64bit_host() {
+cat > $TMPC <<EOF
+#if __SIZEOF_POINTER__ != 8
+#error not 64 bit system
+#endif
+int main(void) { return 0; }
+EOF
+  compile_object "$1"
+}
+
+if check_64bit_host "$CPU_CFLAGS"; then
+    host_bits=64
+else
+    host_bits=32
+fi
+
 if test -n "$host_arch" && {
     ! test -d "$source_path/linux-user/include/host/$host_arch" ||
     ! test -d "$source_path/common-user/host/$host_arch"; }; then
@@ -1028,7 +1047,7 @@  if test "$static" = "yes" ; then
   fi
   plugins="no"
 fi
-if test "$plugins" != "no"; then
+if test "$plugins" != "no" && test $host_bits -eq 64; then
   plugins=yes
   subdirs="$subdirs contrib/plugins"
 fi