I am working on extending coremu (parallel version of qemu). Currently, the code cache in coremu is private, I am working towards to make it shared by all cores. I think the add_tb_jump may not be atomic.
Thanks Xin On Wed, Jan 25, 2012 at 11:22 AM, Peter Maydell <peter.mayd...@linaro.org> wrote: > On 25 January 2012 15:55, Xin Tong <xerox.time.t...@gmail.com> wrote: >> The segfault is caused by jumping to the middle of an instruction. so >> i want to know which TB jumps here. > > (a) Assuming it doesn't take too long to get there, you should > be able to get this information by turning on the debug log > via -d whatever. If it does take too long to get to the crash, > you can use the savevm/loadvm support to save the VM state > at some point slightly before the crash. Then you can run > with the debug log enabled and '-loadvm tag' to start from the > point when you saved the VM state. [disclaimer: depending on the > guest machine you're emulating you might be unlucky and need to > fix save/load first, but that's a worthwhile thing anyway :-)] > > (b) Might not be what you're seeing, but there's a class of bug that > can look like a jump to the middle of an instruction that happens > when: > * step 1: we generate code for a TB > * a load or store within that TB causes an exception > * to get CPUState in sync with the point of the exception > (and in particular to translate the host PC at the exception > into the guest PC at that point) we call cpu_restore_state() > * cpu_restore_state() calls gen_intermediate_code_pc() to > request a retranslation of the TB with extra info to allow > us to do a host-PC-to-guest-PC lookup > * Note that gen_intermediate_code_pc() overwrites the generated > code that already exists in memory, and stops as soon as it > reaches the point of the exception. This is harmless because > we are just rewriting the same bytes to memory that were there > already, but disastrous if... > * ...due to a bug, gen_intermediate_code_pc() generates different > code to that generated back in step 1; this tends to result > in writing half an instruction at the point where it stops > * subsequent attempts to execute that TB tend to result in > weird crashes, often looking like an attempt to jump into > the middle of an instruction > > This is why it's critical that gen_intermediate_code()+TCG always > deterministically generates exactly the same native code every > time. > > -- PMM