"Dr. David Alan Gilbert" <dgilb...@redhat.com> wrote:
> * Juan Quintela (quint...@redhat.com) wrote:
>> Note that there are two reason for this, ARM and PPC do things like
>> guests with 4kb pages on hosts with 16/64kb hosts, and then we have
>> HugePages. Note all the workarounds that postcopy has to do because
>> to work in HugePages size.
>
> There are some fun problems with changing the bitmap page size;
> off the top of my head, the ones I can remember include:
> a) I'm sure I've seen rare cases where a target page is marked as
> dirty inside a hostpage; I'm guessing that was qemu's doing, but
> there are more subtle cases, e.g. running a 4kb guest on a 64kb host;
> it's legal - and 4kb power guests used to exist; I think in those
> cases you see KVM only marking one target page as dirty.
/*
* bitmap-traveling is faster than memory-traveling (for addr...)
* especially when most of the memory is not dirty.
*/
for (i = 0; i < len; i++) {
if (bitmap[i] != 0) {
c = leul_to_cpu(bitmap[i]);
do {
j = ctzl(c);
c &= ~(1ul << j);
page_number = (i * HOST_LONG_BITS + j) * hpratio;
addr = page_number * TARGET_PAGE_SIZE;
ram_addr = start + addr;
cpu_physical_memory_set_dirty_range(ram_addr,
TARGET_PAGE_SIZE * hpratio, clients);
} while (c != 0);
}
}
Thisis the code that we end using when we are synchronizing from kvm, so
if we don't have all pages of a host page set to one (or zero) I think
we are doing something wrong, no? Or I am missunderstanding the code?
> b) Are we required to support migration across hosts of different
> pagesize;
> and if we do that what size should a bit represent?
> People asked about it during postcopy but I think it's restricted to
> matching sizes. I don't think precopy has any requirement for matching
> host pagesize at the moment. 64bit ARM does 4k, 64k and I think 16k
> was
> added later.
With current precopy, we should work independently of the host page size
(famous last words), and in a 1st step, I will only send pages of the
size TARGET_PAGE_SIZE. I will only change the bitmaps. We can add
bigger pages later.
> c) Hugepages have similar issues; precopy doesn't currently have any
> requirement for the hugepage selection on the two hosts to match,
> but it does on postcopy. Also you don't want to have a single dirty
> bit for a 1GB host hugepage if you can handle detecting changes at
> a finer grain level.
I agree here, I was thinking more on the Power/ARM case than the
HugePage case. For the 2MB page, we could think about doing it, for the
1GB case, it is not gonna work.
Later, Juan.
