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deprecation: don't enable TCG plugins by default on 32 bit hosts

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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.

Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20240916085400.1046925-2-alex.bennee@linaro.org>
This commit is contained in:
Alex Bennée 2024-09-16 09:53:43 +01:00
parent 14556211bc
commit cf2a78cbbb
2 changed files with 31 additions and 1 deletions
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21
configure vendored
View File

@ -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

11
docs/about/deprecated.rst
View File

@ -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
--------------------