Re: [RFC PATCH 3/3] x86: Create dma_mark_dirty to dirty pages used for DMA by VM guest

[Michael S. Tsirkin]

On Mon, Dec 14, 2015 at 09:59:13AM -0800, Alexander Duyck wrote:
> On Mon, Dec 14, 2015 at 9:20 AM, Michael S. Tsirkin <mst@xxxxxxxxxx> wrote:
> > On Mon, Dec 14, 2015 at 08:34:00AM -0800, Alexander Duyck wrote:
> >> > This way distro can use a guest agent to disable
> >> > dirtying until before migration starts.
> >>
> >> Right.  For a v2 version I would definitely want to have some way to
> >> limit the scope of this.  My main reason for putting this out here is
> >> to start altering the course of discussions since it seems like were
> >> weren't getting anywhere with the ixgbevf migration changes that were
> >> being proposed.
> >
> > Absolutely, thanks for working on this.
> >
> >> >> +     unsigned long pg_addr, start;
> >> >> +
> >> >> +     start = (unsigned long)addr;
> >> >> +     pg_addr = PAGE_ALIGN(start + size);
> >> >> +     start &= ~(sizeof(atomic_t) - 1);
> >> >> +
> >> >> +     /* trigger a write fault on each page, excluding first page */
> >> >> +     while ((pg_addr -= PAGE_SIZE) > start)
> >> >> +             atomic_add(0, (atomic_t *)pg_addr);
> >> >> +
> >> >> +     /* trigger a write fault on first word of DMA */
> >> >> +     atomic_add(0, (atomic_t *)start);

Actually, I have second thoughts about using atomic_add here,
especially for _sync.

Many architectures do

#define ATOMIC_OP_RETURN(op, c_op)                                      \
static inline int atomic_##op##_return(int i, atomic_t *v)              \
{                                                                       \
        unsigned long flags;                                            \
        int ret;                                                        \
                                                                        \
        raw_local_irq_save(flags);                                      \
        ret = (v->counter = v->counter c_op i);                         \
        raw_local_irq_restore(flags);                                   \
                                                                        \
        return ret;                                                     \
}

and this is not safe if device is still doing DMA to/from
this memory.

Generally, atomic_t is there for SMP effects, not for sync
with devices.

This is why I said you should do
	cmpxchg(pg_addr, 0xdead, 0xdead); 

Yes, we probably never actually want to run m68k within a VM,
but let's not misuse interfaces like this.


> >> >
> >> > start might not be aligned correctly for a cast to atomic_t.
> >> > It's harmless to do this for any memory, so I think you should
> >> > just do this for 1st byte of all pages including the first one.
> >>
> >> You may not have noticed it but I actually aligned start in the line
> >> after pg_addr.
> >
> > Yes you did. alignof would make it a bit more noticeable.
> >
> >>  However instead of aligning to the start of the next
> >> atomic_t I just masked off the lower bits so that we start at the
> >> DWORD that contains the first byte of the starting address.  The
> >> assumption here is that I cannot trigger any sort of fault since if I
> >> have access to a given byte within a DWORD I will have access to the
> >> entire DWORD.
> >
> > I'm curious where does this come from.  Isn't it true that access is
> > controlled at page granularity normally, so you can touch beginning of
> > page just as well?
> 
> Yeah, I am pretty sure it probably is page granularity.  However my
> thought was to try and stick to the start of the DMA as the last
> access.  That way we don't pull in any more cache lines than we need
> to in order to dirty the pages.  Usually the start of the DMA region
> will contain some sort of headers or something that needs to be
> accessed with the highest priority so I wanted to make certain that we
> were forcing usable data into the L1 cache rather than just the first
> cache line of the page where the DMA started.  If however the start of
> a DMA was the start of the page there is nothing there to prevent
> that.

OK, maybe this helps. You should document all these tricks
in code comments.

> >>  I coded this up so that the spots where we touch the
> >> memory should match up with addresses provided by the hardware to
> >> perform the DMA over the PCI bus.
> >
> > Yes but there's no requirement to do it like this from
> > virt POV. You just need to touch each page.
> 
> I know, but at the same time if we match up with the DMA then it is
> more likely that we avoid grabbing unneeded cache lines.  In the case
> of most drivers the data for headers and start is at the start of the
> DMA.  So if we dirty the cache line associated with the start of the
> DMA it will be pulled into the L1 cache and there is a greater chance
> that it may already be prefetched as well.
> 
> >> Also I intentionally ran from highest address to lowest since that way
> >> we don't risk pushing the first cache line of the DMA buffer out of
> >> the L1 cache due to the PAGE_SIZE stride.
> >
> > Interesting. How does order of access help with this?
> 
> If you use a PAGE_SIZE stride you will start evicting things from L1
> cache after something like 8 accesses on an x86 processor as most of
> the recent ones have a 32K 8 way associative L1 cache.  So if I go
> from back to front then I evict the stuff that would likely be in the
> data portion of a buffer instead of headers which are usually located
> at the front.

I see, interesting.

> > By the way, if you are into these micro-optimizations you might want to
> > limit prefetch, to this end you want to access the last line of the
> > page.  And it's probably worth benchmarking a bit and not doing it all just
> > based on theory, keep code simple in v1 otherwise.
> 
> My main goal for now is functional code over high performance code.
> That is why I have kept this code fairly simple.  I might have done
> some optimization but it was as much about the optimization as keeping
> the code simple.

Well you were trying to avoid putting extra stress on
the cache, and it seems clear to me that prefetch
is not your friend here. So
-             atomic_add(0, (atomic_t *)pg_addr);
+             atomic_add(0, (atomic_t *)(pg_addr + PAGE_SIZE - sizeof(atomic_t));
(or whatever we change atomic_t to) is probably a win.

>  For example by using the start of the page instead
> of the end I could easily do the comparison against start and avoid
> doing more than one write per page.

That's probably worth fixing, we don't want two atomics
if we can help it.

-     while ((pg_addr -= PAGE_SIZE) > start)
+     while ((pg_addr -= PAGE_SIZE) >= PAGE_ALIGN(start + PAGE_SIZE))

will do it with no fuss.

> The issue for me doing performance testing is that I don't have
> anything that uses DMA blocks that are actually big enough to make use
> of the PAGE_SIZE stride.  That is why the PAGE_SIZE stride portion is
> mostly just theoretical.  I just have a few NICs and most of them only
> allocate 1 page or so for DMA buffers.  What little benchmarking I
> have done with netperf only showed a ~1% CPU penalty for the page
> dirtying code.  For setups where we did more with the DMA such as
> small packet handling I would expect that value to increase, but I
> still wouldn't expect to see a penalty of much more than ~5% most
> likely since there are still a number of other items that are calling
> atomic operations as well such as the code for releasing pages.
> 
> - Alex
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