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Re: [Qemu-devel] [PATCH v3 04/10] tcg: Init TB's direct jumps before mak
From: |
Alex Bennée |
Subject: |
Re: [Qemu-devel] [PATCH v3 04/10] tcg: Init TB's direct jumps before making it visible |
Date: |
Tue, 19 Apr 2016 14:07:37 +0100 |
User-agent: |
mu4e 0.9.17; emacs 25.0.92.6 |
Sergey Fedorov <address@hidden> writes:
> On 19/04/16 13:55, Alex Bennée wrote:
>> Sergey Fedorov <address@hidden> writes:
>>
>>> From: Sergey Fedorov <address@hidden>
>>>
>>> Initialize TB's direct jump list data fields and reset the jumps before
>>> tb_link_page() puts it into the physical hash table and the physical
>>> page list. So TB is completely initialized before it becomes visible.
>>>
>>> Signed-off-by: Sergey Fedorov <address@hidden>
>>> Signed-off-by: Sergey Fedorov <address@hidden>
>>> ---
>>>
>>> Changes in v2:
>>> * Tweaked a comment
>>>
>>> translate-all.c | 27 ++++++++++++++-------------
>>> 1 file changed, 14 insertions(+), 13 deletions(-)
>>>
>>> diff --git a/translate-all.c b/translate-all.c
>>> index 7ac7916f2792..dfa7f0d64e76 100644
>>> --- a/translate-all.c
>>> +++ b/translate-all.c
>>> @@ -1133,19 +1133,6 @@ static void tb_link_page(TranslationBlock *tb,
>>> tb_page_addr_t phys_pc,
>>> tb->page_addr[1] = -1;
>>> }
>>>
>>> - assert(((uintptr_t)tb & 3) == 0);
>>> - tb->jmp_list_first = (uintptr_t)tb | 2;
>>> - tb->jmp_list_next[0] = (uintptr_t)NULL;
>>> - tb->jmp_list_next[1] = (uintptr_t)NULL;
>>> -
>>> - /* init original jump addresses */
>>> - if (tb->jmp_reset_offset[0] != TB_JMP_RESET_OFFSET_INVALID) {
>>> - tb_reset_jump(tb, 0);
>>> - }
>>> - if (tb->jmp_reset_offset[1] != TB_JMP_RESET_OFFSET_INVALID) {
>>> - tb_reset_jump(tb, 1);
>>> - }
>>> -
>>> #ifdef DEBUG_TB_CHECK
>>> tb_page_check();
>>> #endif
>>> @@ -1254,6 +1241,20 @@ TranslationBlock *tb_gen_code(CPUState *cpu,
>>> ROUND_UP((uintptr_t)gen_code_buf + gen_code_size + search_size,
>>> CODE_GEN_ALIGN);
>>>
>>> + /* init jump list */
>>> + assert(((uintptr_t)tb & 3) == 0);
>>> + tb->jmp_list_first = (uintptr_t)tb | 2;
>>> + tb->jmp_list_next[0] = (uintptr_t)NULL;
>>> + tb->jmp_list_next[1] = (uintptr_t)NULL;
>>> +
>>> + /* init original jump addresses wich has been set during
>>> tcg_gen_code() */
>>> + if (tb->jmp_reset_offset[0] != TB_JMP_RESET_OFFSET_INVALID) {
>>> + tb_reset_jump(tb, 0);
>>> + }
>>> + if (tb->jmp_reset_offset[1] != TB_JMP_RESET_OFFSET_INVALID) {
>>> + tb_reset_jump(tb, 1);
>>> + }
>>> +
>> If we are really concerned about ensuring everything is set before we
>> insert the TB into the list should we not have an explicit write barrier
>> before we call to link the page?
>
> Currently, it is synchronized by 'tb_lock', so no need in a memory
> barrier here. So this is a simple rearrangement of code to a more
> suitable place and maybe just a preparation for relaxing locking scheme
> in future. It would be ahead of time and unnecessary overhead to put a
> barrier in this patch. Do you think it's worth to mention that in the
> commit message?
Good point. Maybe it would be better to clean-up the comment in
tb_link_page() with the assumption that memory consistency for linking
the new TB is either explicit (user mode, tb_lock) or implicit in single
threaded softmmu emulation.
We can then update the comment when we add the MTTCG patches.
>
> Kind regards,
> Sergey
>
>>
>>> /* check next page if needed */
>>> virt_page2 = (pc + tb->size - 1) & TARGET_PAGE_MASK;
>>> phys_page2 = -1;
>>
--
Alex Bennée
[Qemu-devel] [PATCH v3 10/10] tcg: Moderate direct block chaining safety checks in user mode, Sergey Fedorov, 2016/04/10