On Mon, Oct 22, 2012 at 07:09:38PM +0800, Xiao Guangrong wrote:
> On 10/22/2012 05:16 PM, Gleb Natapov wrote:
> > On Fri, Oct 19, 2012 at 03:37:32PM +0800, Xiao Guangrong wrote:
> >> After commit b3356bf0dbb349 (KVM: emulator: optimize "rep ins" handling),
> >> the pieces of io data can be collected and write them to the guest memory
> >> or MMIO together.
> >>
> >> Unfortunately, kvm splits the mmio access into 8 bytes and store them to
> >> vcpu->mmio_fragments. If the guest uses "rep ins" to move large data, it
> >> will cause vcpu->mmio_fragments overflow
> >>
> >> The bug can be exposed by isapc (-M isapc):
> >>
> >> [23154.818733] general protection fault: 0000 [#1] SMP DEBUG_PAGEALLOC
> >> [ ......]
> >> [23154.858083] Call Trace:
> >> [23154.859874] [<ffffffffa04f0e17>] kvm_get_cr8+0x1d/0x28 [kvm]
> >> [23154.861677] [<ffffffffa04fa6d4>] kvm_arch_vcpu_ioctl_run+0xcda/0xe45 [kvm]
> >> [23154.863604] [<ffffffffa04f5a1a>] ? kvm_arch_vcpu_load+0x17b/0x180 [kvm]
> >>
> >>
> >> Actually, we can use one mmio_fragment to store a large mmio access for the
> >> mmio access is always continuous then split it when we pass the mmio-exit-info
> >> to userspace. After that, we only need two entries to store mmio info for
> >> the cross-mmio pages access
> >>
> > I wonder can we put the data into coalesced mmio buffer instead of
>
> If we put all mmio data into coalesced buffer, we should:
> - ensure the userspace program uses KVM_REGISTER_COALESCED_MMIO to register
> all mmio regions.
>
It appears to not be so.
Userspace calls kvm_flush_coalesced_mmio_buffer() after returning from
KVM_RUN which looks like this:
void kvm_flush_coalesced_mmio_buffer(void)
{
KVMState *s = kvm_state;
if (s->coalesced_flush_in_progress) {
return;
}
s->coalesced_flush_in_progress = true;
if (s->coalesced_mmio_ring) {
struct kvm_coalesced_mmio_ring *ring = s->coalesced_mmio_ring;
while (ring->first != ring->last) {
struct kvm_coalesced_mmio *ent;
ent = &ring->coalesced_mmio[ring->first];
cpu_physical_memory_write(ent->phys_addr, ent->data, ent->len);
smp_wmb();
ring->first = (ring->first + 1) % KVM_COALESCED_MMIO_MAX;
}
}
s->coalesced_flush_in_progress = false;
}
Nowhere in this function we check that MMIO region was registered with
KVM_REGISTER_COALESCED_MMIO. We do not even check that the address is
MMIO.
> - even if the MMIO region is not used by emulated-device, it also need to be
> registered.
Same. I think writes to non registered region will be discarded.
>
> It will breaks old version userspace program.
>
> > exiting for each 8 bytes? Is it worth the complexity?
>
> Simpler way is always better but i failed, so i appreciate your guys comments.
>
Why have you failed? Exiting for each 8 bytes is infinitely better than
buffer overflow. My question about complexity was towards theoretically
more complex code that will use coalesced MMIO buffer.
--
Gleb.
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