Re: Does the page boundary check still necessary?

[Richard Henderson <richard.henderson@linaro.org>]

On 2/15/23 16:28, Kenneth Lee wrote:
> On Wed, Feb 15, 2023 at 04:26:18PM -1000, Richard Henderson wrote:
>> Date: Wed, 15 Feb 2023 16:26:18 -1000
>> From: Richard Henderson <richard.henderson@linaro.org>
>> To: Kenneth Lee <Kenneth-Lee-2012@foxmail.com>
>> Cc: qemu-devel@nongnu.org
>> Subject: Re: Does the page boundary check still necessary?
>> User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101
>>   Thunderbird/102.7.1
>>
>> On 2/15/23 15:45, Kenneth Lee wrote:
>>>>> Now the chained TBs have been link with tb_link_page(), the chain will
>>>>> be rebuilt if it is invalidate on page. So why is this check still there?
>>>>
>>>> Even for a guest which doesn't use paging, and therefore does not need to
>>>> worry about memory maps changing, we still enable breakpoints and
>>>> watchpoints on a per-page basis.
>>>>
>>>
>>> Thank you. So is this the only reason? May I write a fine grained
>>> checking to remove this limitation?
>>
>> No.
>>
> Why?

When breakpoints change, we discard all translations on the affected page, similarly to 
how we handle writes from self-modifying code.  If you link from further away, then TBs 
won't be invalidated properly when breakpoints change.  For most guests, this isn't a 
limitation because we also have to care for modifications to page tables, so we can't 
allow such links anyway.

I have no idea what you're trying to accomplish that's different from existing guests.


r~