Messages in this thread | | | Date | Wed, 29 Feb 2012 19:16:23 +0900 | From | Takuya Yoshikawa <> | Subject | Re: [PATCH 3/4] KVM: Switch to srcu-less get_dirty_log() |
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Avi Kivity <avi@redhat.com> 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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