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Re: [Qemu-devel] [PATCH 1/8] tcg: Clean up direct block chaining data fi
From: |
Sergey Fedorov |
Subject: |
Re: [Qemu-devel] [PATCH 1/8] tcg: Clean up direct block chaining data fields |
Date: |
Thu, 24 Mar 2016 17:02:31 +0300 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.6.0 |
On 24/03/16 16:42, Alex Bennée wrote:
>> diff --git a/include/exec/exec-all.h b/include/exec/exec-all.h
>> > index 05a151da4a54..cc3d2ca25917 100644
>> > --- a/include/exec/exec-all.h
>> > +++ b/include/exec/exec-all.h
>> > @@ -257,20 +257,32 @@ struct TranslationBlock {
>> > struct TranslationBlock *page_next[2];
>> > tb_page_addr_t page_addr[2];
>> >
>> > - /* the following data are used to directly call another TB from
>> > - the code of this one. */
>> > - uint16_t tb_next_offset[2]; /* offset of original jump target */
>> > + /* The following data are used to directly call another TB from
>> > + * the code of this one. This can be done either by emitting direct or
>> > + * indirect native jump instructions. These jumps are reset so that
>> > the TB
>> > + * just continue its execution. The TB can be linked to another one by
>> > + * setting one of the jump targets (or patching the jump
>> > instruction). Only
>> > + * two of such jumps are supported.
>> > + */
>> > + uint16_t jmp_reset_offset[2]; /* offset of original jump target */
>> > +#define TB_JMP_RESET_OFFSET_INVALID 0xffff /* indicates no jump generated
>> > */
>> > #ifdef USE_DIRECT_JUMP
>> > - uint16_t tb_jmp_offset[2]; /* offset of jump instruction */
>> > + uint16_t jmp_insn_offset[2]; /* offset of native jump instruction */
>> > #else
>> > - uintptr_t tb_next[2]; /* address of jump generated code */
>> > + uintptr_t jmp_target_addr[2]; /* target address for indirect jump */
>> > #endif
>> > - /* list of TBs jumping to this one. This is a circular list using
>> > - the two least significant bits of the pointers to tell what is
>> > - the next pointer: 0 = jmp_next[0], 1 = jmp_next[1], 2 =
>> > - jmp_first */
>> > - struct TranslationBlock *jmp_next[2];
>> > - struct TranslationBlock *jmp_first;
>> > + /* Each TB has an assosiated circular list of TBs jumping to this one.
>> > + * jmp_list_first points to the first TB jumping to this one.
>> > + * jmp_list_next is used to point to the next TB in a list.
>> > + * Since each TB can have two jumps, it can participate in two lists.
>> > + * The two least significant bits of a pointer are used to choose
>> > which
>> > + * data field holds a pointer to the next TB:
>> > + * 0 => jmp_list_next[0], 1 => jmp_list_next[1], 2 => jmp_list_first.
>> > + * In other words, 0/1 tells which jump is used in the pointed TB,
>> > + * and 2 means that this is a pointer back to the target TB of this
>> > list.
>> > + */
>> > + struct TranslationBlock *jmp_list_next[2];
>> > + struct TranslationBlock *jmp_list_first;
> OK I found that tricky to follow. Where does the value of the pointer
> come from that sets these bottom bits? The TB jumping to this TB sets it?
Yeah, that's not easy to describe. Initially, we set:
tb->jmp_list_first = tb | 2
That makes an empty list: jmp_list_first just points to the this TB and
the low bits are 2.
After that we can add a TB to the list in tb_add_jump():
tb->jmp_list_next[n] = tb_next->jmp_list_first;
tb_next->jmp_list_first = tb | n;
where 'tb' is going to jump to 'tb_next', 'n' (can be 0 or 1) is an
index of jump target of 'tb'.
(I simplified the code here)
Any ideas how to make it more clear in the comment?
Kind regards,
Sergey
- [Qemu-devel] [PATCH 0/8] tcg: Direct block chaining clean-up, sergey . fedorov, 2016/03/24
- [Qemu-devel] [PATCH 1/8] tcg: Clean up direct block chaining data fields, sergey . fedorov, 2016/03/24
- Re: [Qemu-devel] [PATCH 1/8] tcg: Clean up direct block chaining data fields, Alex Bennée, 2016/03/24
- Re: [Qemu-devel] [PATCH 1/8] tcg: Clean up direct block chaining data fields,
Sergey Fedorov <=
- Re: [Qemu-devel] [PATCH 1/8] tcg: Clean up direct block chaining data fields, Alex Bennée, 2016/03/24
- Re: [Qemu-devel] [PATCH 1/8] tcg: Clean up direct block chaining data fields, Sergey Fedorov, 2016/03/24
- Re: [Qemu-devel] [PATCH 1/8] tcg: Clean up direct block chaining data fields, Paolo Bonzini, 2016/03/24
- Re: [Qemu-devel] [PATCH 1/8] tcg: Clean up direct block chaining data fields, Alex Bennée, 2016/03/24
- Re: [Qemu-devel] [PATCH 1/8] tcg: Clean up direct block chaining data fields, Richard Henderson, 2016/03/28
- Re: [Qemu-devel] [PATCH 1/8] tcg: Clean up direct block chaining data fields, Paolo Bonzini, 2016/03/29
- Re: [Qemu-devel] [PATCH 1/8] tcg: Clean up direct block chaining data fields, Paolo Bonzini, 2016/03/29
- Re: [Qemu-devel] [PATCH 1/8] tcg: Clean up direct block chaining data fields, Sergey Fedorov, 2016/03/29
- Re: [Qemu-devel] [PATCH 1/8] tcg: Clean up direct block chaining data fields, Richard Henderson, 2016/03/29
- Re: [Qemu-devel] [Qemu-arm] [PATCH 1/8] tcg: Clean up direct block chaining data fields, Peter Maydell, 2016/03/29