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

[Peter Xu]

On Fri, Dec 11, 2020 at 09:13:10AM +0800, zhukeqian wrote:
> 
> 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?

Yes we can, but I think it's not strongly necessary.  As Zenghui pointed out,
the callers of memory_region_clear_dirty_bitmap() should always be aware of the
fact that dirty bitmap is always page size based.

OTOH I'm more worried on the other question on how we handle guest psize !=
host psize case for migration now...

Thanks,

-- 
Peter Xu