Re: [PATCH] kvm tools: Use mmap for working with disk image V2

[Avi Kivity <avi@redhat.com>]

On 04/10/2011 11:30 AM, Alexander Graf wrote:
> On 10.04.2011, at 10:15, Avi Kivity wrote:
>
> >  On 04/09/2011 03:20 PM, Alexander Graf wrote:
> >>  On 09.04.2011, at 14:14, Sasha Levin wrote:
> >>
> >>  >   Attempt to use mmap first for working with a disk image, if the attempt is failed (for example, large image on a 32bit system) fallback to using read/write.
> >>
> >>  That reminds me of an idea I had quite a while back.
> >>
> >>  What if we mmap'ed a raw disk image directly into the guest's address space? This could for example be done through a virtio feature addition, keeping the disk accessible through normal virtio plus the mmap'ed part. At least in writeback mode, this should perform pretty well, as we'd save all the userspace exits. It'd basically be almost like vhost-blk :).
> >>
> >>  Have you thought about trying out to implement such a feature?
> >
> >  A creative idea, but I don't think it will work.  On EPT hosts we don't have accessed/dirty bits so you have to incur at least write faults to track dirty data and perhaps read faults to gather recency information.  On non-EPT you have to scan page tables to find out what you have to write out, and flush TLBs.  Cache misses, which you'd expect there to be quite a few, would stall the vcpu (unless you use asynchronous page faults) and contribute less information to the host than virtio-blk (location of access but not size).  Write misses are converted to read-modify-write operations.
>
> Since we're moving the 4k sector sizes, the RMW argument shouldn't matter too much in a couple of years from now.

I wasn't talking about the sector size, rather that

   memcpy(&mmapped_disk[sector * SECTOR_SIZE], data, SECTOR_SIZE)

writes the data word by word, so on the first write you have to read in 
the entire page, then modify the first and following words.

> As for the faults, yes. We'd basically have to declare the file region as dirty logged, which means we get lots of page faults when accessing them. However, these are all lightweight exits. So we take one lightweight exit for each 4k chunk. when doing writes. For reads, we probably really do need asynchronous page faults - everything else would stall the vcpus way too long.

These are still very expensive, compared to virtio-blk which can get you 
megabytes worth of data with a single exit.

-- 
error compiling committee.c: too many arguments to function