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From: | Anthony Liguori |
Subject: | Re: [Qemu-devel] [PATCH] Inter-VM shared memory PCI device |
Date: | Tue, 09 Mar 2010 15:44:58 -0600 |
User-agent: | Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.1.5) Gecko/20091209 Fedora/3.0-4.fc12 Lightning/1.0pre Thunderbird/3.0 |
On 03/08/2010 07:16 AM, Avi Kivity wrote:
On 03/08/2010 03:03 PM, Paul Brook wrote:In a cross environment that becomes extremely hairy. For example the x86 architecture effectively has an implicit write barrier before every store, andOn 03/08/2010 12:53 AM, Paul Brook wrote:Support an inter-vm shared memory device that maps a shared-memory object as a PCI device in the guest. This patch also supports interrupts between guest by communicating over a unix domain socket. This patch applies to the qemu-kvm repository.No. All new devices should be fully qdev based.I suspect you've also ignored a load of coherency issues, especially whennot using KVM. As soon as you have shared memory in more than one host thread/process you have to worry about memory barriers.Shouldn't it be sufficient to require the guest to issue barriers (and to ensure tcg honours the barriers, if someone wants this with tcg)?.an implicit read barrier before every load.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.
Shared memory only makes sense when using KVM. In fact, we should actively disable the shared memory device when not using KVM.
Regards, Anthony Liguori
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