On 01/22/2010 04:22 AM, Sheng Yang wrote:
The default action of coalesced MMIO is, cache the writing in buffer, until:
1. The buffer is full.
2. Or the exit to QEmu due to other reasons.
But this would result in a very late writing in some condition.
1. The each time write to MMIO content is small.
2. The writing interval is big.
3. No need for input or accessing other devices frequently.
This issue was observed in a experimental embbed system. The test image
simply print "test" every 1 seconds. The output in QEmu meets expectation,
but the output in KVM is delayed for seconds.
Per Avi's suggestion, I hooked a flushing for coalesced MMIO buffer in VGA
update handler. By this way, We don't need vcpu explicit exit to QEmu to
handle this issue.
Signed-off-by: Sheng Yang<sheng@xxxxxxxxxxxxxxx>
---
Like this?
qemu-kvm.c | 26 ++++++++++++++++++++++++--
qemu-kvm.h | 6 ++++++
vl.c | 2 ++
3 files changed, 32 insertions(+), 2 deletions(-)
+#ifdef KVM_CAP_COALESCED_MMIO
+void kvm_flush_coalesced_mmio_buffer(void)
+{
+ if (kvm_state->coalesced_mmio_ring) {
+ struct kvm_coalesced_mmio_ring *ring =
+ kvm_state->coalesced_mmio_ring;
+ while (ring->first != ring->last) {
+ cpu_physical_memory_rw(ring->coalesced_mmio[ring->first].phys_addr,
+&ring->coalesced_mmio[ring->first].data[0],
+ ring->coalesced_mmio[ring->first].len, 1);
+ smp_wmb();
+ ring->first = (ring->first + 1) % KVM_COALESCED_MMIO_MAX;
+ }
+ }
+}
+#endif
+
qemu has a coalesced mmio api, (qemu_register_colaesced_mmio), so please
follow it (stubs in exec.c, and implementation in kvm-specific files).
Please send the patch against the uq/master branch (which follow upstream).
diff --git a/vl.c b/vl.c
index 9edea10..64902f2 100644
--- a/vl.c
+++ b/vl.c
@@ -3235,6 +3235,7 @@ static void gui_update(void *opaque)
interval = dcl->gui_timer_interval;
dcl = dcl->next;
}
+ kvm_flush_coalesced_mmio_buffer();
qemu_mod_timer(ds->gui_timer, interval + qemu_get_clock(rt_clock));
}
Better to do that before the call to dpy_refresh().
@@ -3242,6 +3243,7 @@ static void nographic_update(void *opaque)
{
uint64_t interval = GUI_REFRESH_INTERVAL;
+ kvm_flush_coalesced_mmio_buffer();
qemu_mod_timer(nographic_timer, interval + qemu_get_clock(rt_clock));
}
Any need to do it here?
(why does nographic_update use a timer?)
--
Do not meddle in the internals of kernels, for they are subtle and quick to panic.
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