Re: [ANNOUNCE] kvm-46 release

On 10/10/07, Avi Kivity [off-list ref] wrote:

Jun Koi wrote:

quoted

On 10/10/07, Avi Kivity [off-list ref] wrote:

quoted

Jun Koi wrote:

quoted

Hi,

On 10/10/07, Avi Kivity [off-list ref] wrote:

quoted

We've now switched to allocating guest memory in userspace rather than
in the kernel.  This is important if you have a mainframe, but also if
you want to share memory between guests and implement nice features like
swapping.

This is interesting! But how can we do that now? (share memory between guests)

It's not exposed by qemu, but you can now mmap() some file (or use SysV
shared memory) and use that as guest memory.

OK, lets say we have 2 guest VMs share a memory, like mmap() a tmpfs
file (which is actually in memory). Now one writes to the memory
(shared file). Can we guarantee that the memory immediately updated,
so other will see the change immediately? Or the data might be cached
for a while, befere being flushed to the shared memory?

Changes are visible immediately.

OK, that is about sharing memory file. How about a shared file in
which a file is a real physical file on the host?

As far as I remember, there is a caution about sharing a file using
mmap() between 2 processes, that is: if the first process modifies the
shared file, it might take a while for the changes to be flushed from
cache to the file. That means the other process doesnt see the change
immediately.

I guess this problem affects us in this case, when we use a real
(physical) file to share between 2 VMs, correct?

Thanks,
Jun

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