On 11/28/2013 11:19 AM, Gleb Natapov wrote:
On Thu, Nov 28, 2013 at 09:55:42AM +0100, Paolo Bonzini wrote:
Il 28/11/2013 07:27, Zhanghaoyu (A) ha scritto:
Without synchronize_rcu you could have
VCPU writes to routing table
e = entry from IRQ routing table
kvm_irq_routing_update(kvm, new);
VCPU resumes execution
kvm_set_msi_irq(e, &irq);
kvm_irq_delivery_to_apic_fast();
where the entry is stale but the VCPU has already resumed execution.
If we use call_rcu()(Not consider the problem that Gleb pointed out
temporarily) instead of synchronize_rcu(), should we still ensure this?
The problem is that we should ensure this, so using call_rcu is not
possible (even not considering the memory allocation problem).
Not changing current behaviour is certainly safer, but I am still not 100%
convinced we have to ensure this.
Suppose guest does:
1: change msi interrupt by writing to pci register
2: read the pci register to flush the write
3: zero idt
I am pretty certain that this code can get interrupt after step 2 on real HW,
but I cannot tell if guest can rely on it to be delivered exactly after
read instruction or it can be delayed by couple of instructions. Seems to me
it would be fragile for an OS to depend on this behaviour. AFAIK Linux does not.
Linux is safe, it does interrupt migration from within the interrupt
handler. If you do that before the device-specific EOI, you won't get
another interrupt until programming the MSI is complete.
Is virtio safe? IIRC it can post multiple interrupts without guest acks.
Using call_rcu() is a better solution than srcu IMO. Less code changes,
consistently faster.
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