Re: [PATCH] kvm: Take into account the unaligned section size when preparing bitmap

[zhukeqian]

On 2020/12/10 22:50, Peter Xu wrote:
> On Thu, Dec 10, 2020 at 10:53:23AM +0800, zhukeqian wrote:
>>
>>
>> On 2020/12/10 10:08, Peter Xu wrote:
>>> Keqian,
>>>
>>> On Thu, Dec 10, 2020 at 09:46:06AM +0800, zhukeqian wrote:
>>>> Hi,
>>>>
>>>> I see that if start or size is not PAGE aligned, it also clears areas
>>>> which beyond caller's expectation, so do we also need to consider this?
>>>
>>> Could you elaborate?
>>>
>>> If start_delta != 0, kvm_log_clear_one_slot() should already go the slow 
>>> path.
>>>
>>> Thanks,
>>>
>>
>> Hi Peter,
>>
>> start_delta /= psize;
>>
>> If start is not PAGE aligned, then start_delta is not PAGE aligned.
>> so I think the above code will implicitly extend our start to be PAGE 
>> aligned.
>>
>> I suggest that we should shrink the start and (start + size) to be PAGE 
>> aligned
>> at beginning of this function.
> 
> Callers should be with TARGET_PAGE_SIZE aligned on the size, so at least 
> x86_64
> should be pretty safe since host/guest page sizes match.
> 
> Though indeed I must confess I don't know how it worked in general when host
> page size != target page size, at least for migration.  For example, I believe
> kvm dirty logging is host page size based, though migration should be 
> migrating
> pages in guest page size granule when it spots a dirty bit set.
> 
Hi,

Indeed, we handle target_page_size aligned @start and @size in general. Maybe 
we'd better
add explicit function comments about alignment requirement, and explicit 
alignment assert
on @start and @size?

Keqian.