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Re: [Qemu-devel] [PATCH 4/5] tcg: reorder removal from lists in tb_phys_
From: |
Paolo Bonzini |
Subject: |
Re: [Qemu-devel] [PATCH 4/5] tcg: reorder removal from lists in tb_phys_invalidate |
Date: |
Mon, 28 Mar 2016 23:21:21 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.6.0 |
On 28/03/2016 17:18, Sergey Fedorov wrote:
> The use pattern of 'tb_invalidated_flag' is a bit intricate; correct me,
> if I'm wrong about the following. Basically, 'tb_invalidated_flag' was
> meant to catch two events:
> * some TB has been invalidated by tb_phys_invalidate();
This is patch 4.
> * the whole translation buffer has been flushed by tb_flush().
This is patch 5.
> Then it is checked to ensure:
> * the last executed TB can be safely patched to directly call the next
> one in cpu_exec();
> * the original TB should be provided for further possible invalidation
> along with the temporarily generated TB when in cpu_exec_nocache().
>
> [...] I would suggest the following solution:
> (1) Use 'tb->pc' as an indicator of whether TB is valid; check for it
> in cpu_exec() when deciding on whether to patch the last executed
> TB or not
> (2) Use 'tcg_ctx.tb_ctx.tb_flush_count' to check for translation buffer
> flushes; capture it before calling tb_gen_code() and compare to it
> afterwards to check if tb_flush() has been called in between
Of course that would work, but it would be slower. I think it is
unnecessary for two reasons:
1) There are two calls to cpu_exec_nocache. One exits immediately with
"break;", the other always sets "next_tb = 0;". Therefore it is safe in
both cases for cpu_exec_nocache to hijack cpu->tb_invalidated_flag.
2) if it were broken, it would _also_ be broken before these patches
because cpu_exec_nocache always runs with tb_lock taken. So I think
documenting the assumptions is better than changing them at the same
time as doing other changes.
Your observation that tb->pc==-1 is not necessarily safe still holds of
course. Probably the best thing is an inline that can do one of:
1) set cs_base to an invalid value (anything nonzero is enough except on
x86 and SPARC; SPARC can use all-ones)
2) sets the flags to an invalid combination (x86 can use all ones)
3) sets the PC to an invalid value (no one really needs it)
Paolo
- [Qemu-devel] [PATCH 0/5] tcg: Misc clean-up patches from Paolo, sergey . fedorov, 2016/03/17
- [Qemu-devel] [PATCH 4/5] tcg: reorder removal from lists in tb_phys_invalidate, sergey . fedorov, 2016/03/17
- Re: [Qemu-devel] [PATCH 4/5] tcg: reorder removal from lists in tb_phys_invalidate, Paolo Bonzini, 2016/03/17
- Re: [Qemu-devel] [PATCH 4/5] tcg: reorder removal from lists in tb_phys_invalidate, Sergey Fedorov, 2016/03/17
- Re: [Qemu-devel] [PATCH 4/5] tcg: reorder removal from lists in tb_phys_invalidate, Sergey Fedorov, 2016/03/28
- Re: [Qemu-devel] [PATCH 4/5] tcg: reorder removal from lists in tb_phys_invalidate,
Paolo Bonzini <=
- Re: [Qemu-devel] [PATCH 4/5] tcg: reorder removal from lists in tb_phys_invalidate, Sergey Fedorov, 2016/03/29
- Re: [Qemu-devel] [PATCH 4/5] tcg: reorder removal from lists in tb_phys_invalidate, Paolo Bonzini, 2016/03/29
- Re: [Qemu-devel] [PATCH 4/5] tcg: reorder removal from lists in tb_phys_invalidate, Sergey Fedorov, 2016/03/29
- Re: [Qemu-devel] [PATCH 4/5] tcg: reorder removal from lists in tb_phys_invalidate, Alex Bennée, 2016/03/29
Re: [Qemu-devel] [PATCH 4/5] tcg: reorder removal from lists in tb_phys_invalidate, Sergey Fedorov, 2016/03/28
[Qemu-devel] [PATCH 3/5] tcg: always keep jump target and tb->jmp_next consistent, sergey . fedorov, 2016/03/17