On Thu, May 02, 2013 at 11:22:52AM -0700, David Daney wrote:
> Hi,
>
> I am working on the MIPS KVM port, and am trying to figure out under
> which circumstances do I need to srcu_read_lock()/srcu_read_unlock()
> the kvm->srcu.
For x86: kvm->srcu protects memory slot information (kvm->memslots) and
in-kernel MMIO/PIO address->device structure mapping (kvm->buses).
Search for synchronize_srcu_expedited() in virt/kvm/ to locate the
updaters.
> I am looking at implementing something similar to arch/x86/kvm/x86.c
> at __msr_io(), where we see:
>
> .
> .
> .
> idx = srcu_read_lock(&vcpu->kvm->srcu);
> for (i = 0; i < msrs->nmsrs; ++i)
> if (do_msr(vcpu, entries[i].index, &entries[i].data))
> break;
> srcu_read_unlock(&vcpu->kvm->srcu, idx);
> .
> .
> .
>
> Why is the srcu_read_lock() taken here? I see no srcu_dereference()
> in the code path that would indicate the need for obtaining the
> lock.
case KVM_GET_MSRS:
r = msr_io(vcpu, argp, kvm_get_msr, 1);
break;
case KVM_SET_MSRS:
r = msr_io(vcpu, argp, do_set_msr, 0);
break;
to
int kvm_get_msr(struct kvm_vcpu *vcpu, u32 msr_index, u64 *pdata)
{
return kvm_x86_ops->get_msr(vcpu, msr_index, pdata);
}
Somewhere down that path memslot information must be accessed.
> I have a feeling that I am missing some essential concept about the
> design of this code, but I don't know what it is.
>
> Can someone explain what is happening here?
For x86 the usage is:
VCPU CODE PATH
--------------
IOCTL(KVM_FD, KVM_VCPU_RUN)
ENTER-KERNEL
SRCU_READ_LOCK()
... large parte of vcpu context code performed with srcu lock held,
so that memory slot information can be used to access guest memory
(gfn_to_memslot for example).
SRCU_READ_UNLOCK()
VMENTER
while in guest mode srcu is not held so that updaters can make
progress
VMEXIT
SRCU_READ_LOCK()
back to vcpu context code
SRCU_READ_UNLOCK before return to userspace.
Also, when emulating HALT (kvm_vcpu_block) srcu is dropped.
UPDATERS
--------
See synchronize_srcu_expedited in virt/kvm/
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