On 10/24/24 09:25, Richard Henderson wrote:
On 10/23/24 23:20, Pierrick Bouvier wrote:
When instrumenting memory accesses for plugin, we force memory accesses
to use the slow path for mmu. [1]
This create a situation where we end up calling ptw_setl_slow.
Since this function gets called during a cpu_exec, start_exclusive then
hangs. This exclusive section was introduced initially for security
reasons [2].
I suspect this code path was never triggered, because ptw_setl_slow
would always be called transitively from cpu_exec, resulting in a hang.
[1]
https://gitlab.com/qemu-project/qemu/-/commit/6d03226b42247b68ab2f0b3663e0f624335a4055
[2] https://gitlab.com/qemu-project/qemu/-/issues/279
Fixes: https://gitlab.com/qemu-project/qemu/-/issues/2566
Signed-off-by: Pierrick Bouvier <pierrick.bouv...@linaro.org>
Oh, wow. I believe this will be fixed by
https://lore.kernel.org/qemu-devel/20241023033432.1353830-19-richard.hender...@linaro.org/
which is in a pending PR.
It might the issue by not triggering the situation we observed.
However, we still have a hidden dead code path where start_exclusive is
called from cpu_exec, not being related to the plugins.
r~
---
target/i386/tcg/sysemu/excp_helper.c | 4 ++++
1 file changed, 4 insertions(+)
diff --git a/target/i386/tcg/sysemu/excp_helper.c
b/target/i386/tcg/sysemu/excp_helper.c
index 8fb05b1f531..f30102b5362 100644
--- a/target/i386/tcg/sysemu/excp_helper.c
+++ b/target/i386/tcg/sysemu/excp_helper.c
@@ -108,6 +108,9 @@ static bool ptw_setl_slow(const PTETranslate *in, uint32_t
old, uint32_t new)
{
uint32_t cmp;
+ /* We are in cpu_exec, and start_exclusive can't be called directly.*/
+ g_assert(current_cpu && current_cpu->running);
+ cpu_exec_end(current_cpu);
/* Does x86 really perform a rmw cycle on mmio for ptw? */
start_exclusive();
cmp = cpu_ldl_mmuidx_ra(in->env, in->gaddr, in->ptw_idx, 0);
@@ -115,6 +118,7 @@ static bool ptw_setl_slow(const PTETranslate *in, uint32_t
old, uint32_t new)
cpu_stl_mmuidx_ra(in->env, in->gaddr, new, in->ptw_idx, 0);
}
end_exclusive();
+ cpu_exec_start(current_cpu);
return cmp == old;
}
