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

[Peter Xu <peterx@redhat.com>]

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.

-- 
Peter Xu