Re: [Qemu-devel] [PATCH] Inter-VM shared memory PCI device

[Avi Kivity <avi@redhat.com>]

On 03/09/2010 11:44 PM, Anthony Liguori wrote:
>> Ah yes.  For cross tcg environments you can map the memory using mmio 
>> callbacks instead of directly, and issue the appropriate barriers there.
>
>
> Not good enough unless you want to severely restrict the use of shared 
> memory within the guest.
>
> For instance, it's going to be useful to assume that you atomic 
> instructions remain atomic.  Crossing architecture boundaries here 
> makes these assumptions invalid.  A barrier is not enough.

You could make the mmio callbacks flow to the shared memory server over 
the unix-domain socket, which would then serialize them.  Still need to 
keep RMWs as single operations.  When the host supports it, implement 
the operation locally (you can't render cmpxchg16b on i386, for example).

> Shared memory only makes sense when using KVM.  In fact, we should 
> actively disable the shared memory device when not using KVM.

Looks like that's the only practical choice.

-- 
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