Avi Kivity <avi@xxxxxxxxxx> wrote:
> Okay. But note you don't need the alignment check; simply allocate the
> array aligned, and a multiple of 16 bytes, in the first place.
OKay, then we can do something like:
for each (x = bitmap[i], y = bitmap[i+1])
if (!x && !y)
continue
else if ...
cmpxchg16b or 8b
...
I will try later.
> > In addition, we should care about the new API. It is not decided about
> > what kind of range can be ordered. I think restricting the range to be
> > long size aligned is natural. Do you have any plan?
>
> Not really. But the current changes are all internal and don't affect
> the user API.
Then I will do my best to improve the performance now!
> > __ffs is really fast compared to other APIs.
>
> You could do something like this
>
> for (i = 0; i < bitmap_size_in_longs; ++i) {
> mask = bitmap[i];
> if (!mask)
> continue;
> for (j = __ffs(mask); mask; mask &= mask - 1, j = __ffs(mask))
> handle i * BITS_PER_LONG + j;
> }
>
> This gives you the speed of __ffs() but without working on zero bits.
Yes, I will do this in v2.
> > We may see partial display updates if we do not hold the mmu_lock during
> > xchg loop: it is possible that pages near the end of the framebuffer alone
> > gets updated sometimes - I noticed this problem when I fixed the TLB flush
> > issue.
>
> I don't understand why this happens.
Because only mmu_lock protects the bitmap for VGA.
xchg i = 1
xchg i = 2
...
xchg i = N
We cannot get a complete snapshot without mmu_lock; if the guest faults on
the Nth page during xchg'ing i = 1, 2, ... then the i = N alone will
become newer.
But others will be updated by the next call, so the problem is restricted:
maybe not noticeable.
> > Not a big problem but still maybe-noticeable change, so I think we should
> > do it separately with some comments if needed.
>
> Well if it's noticable on the framebuffer it's also noticable with live
> migration. We could do it later, but we need to really understand it first.
About live migration, we do not mind whether the bitmap is a complete snapshot.
In addition, we cannot do anything because the emulator can access the bitmap
without mmu_lock.
What we are doing is calling GET_DIRTY_LOG slot by slot: so already the
result is not a snapshot at all.
In the end, at the last stage, we will stop the guest and get a complete snapshot.
> > In addition, we do not want to scan the dirty bitmap twice. Using the
> > bits value soon after it is read into a register seems to be the fastest.
>
> Probably.
>
> > BTW, I also want to decide the design of the new API at this chance.
>
> Let's wait with that. We're adding APIs too quickly. Let's squeeze
> everything we can out of the current APIs.
I agree with you of course.
At the same time, we cannot say anything without actually implementing
sample userspace programs.
So I want to see how much improvement the proposed API can achieve.
I thought this might be a good GSoC project but ...
Takuya
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