[kvm-devel] [PATCH][RFC] Improve PIO latency

[Anthony Liguori]

The attached patch modifies libkvmctl to only make SET_REG/GET_REG 
ioctls when needed for PIO instructions.  I was only able to do this for 
out instructions because I didn't want to break the kernel ABI.

I think we should change the API though so we can do this with other 
types of IO instructions.  Before this patch, the time line for a PIO 
instruction looks something like this:

All times in nanoseconds and are round trips from the guests perspective 
for an out instruction on an AMD X2 4200.

1015 - immediately after restoring saving guest registers
1991 - handled within the kernel in io_interception
2294 - libkvmctl returns immediately
2437 - w/ patch
3311 - w/o patch

The first data point is the best we could possible do.  The only work 
being done after the VMRUN is a VMSAVE/VMLOAD, saving the guest 
registers, and restoring the host registers.  The VMSAVE/VMLOAD is 
needed so that vmcb->save.eip can be updated.[1]  I played around 
reducing the register savings but the differences weren't noticable.

I suspect that more intelligent handling of things like FPU save/restore 
should be able to reduce the second data point.  This will also improve 
some other exit paths (like shadow paging).  We save/restore an awful 
lot of state considering that we probably return back to the guest for 
the vast majority of exits.

On this system, a sysenter based syscall is roughly 100 nsec so I'm 
pretty happy with the third data point.  This is just what one would expect.

With the attached patch, we reduce the time we spend in QEMU by 
eliminating unnecessary ioctl()s.  This cuts down the total trip time by 
about 1/3.  We should be able to do this for in{b,w,l}s too.

With these patches, we get an improvement in disk performance.

virtbench can measure disk latency and small disk read speeds (16kb). 
According to virt bench:

w/o patch
80% of native - latency
61% of native - bandwidth

w/ patch
96% of native - latency
99% of native - bandwidth

Before getting too excited, we're still only 25% of native with dbench. 
   We see a small improvement with the patch (around 10%) but there's 
an awful lot of variability.

There are quite a few things that should improve disk performance in 
QEMU.  Moving to an asynchronous IO model (QEMU CVS), and utilizing 
linux-aio should make a pretty significant difference.

The last interesting bit is that the native latency for an IDE PIO 
operation is around 750 nsec on this system.  Theoretically, we should 
be able to get pretty close to native IDE performance with emulation. 
At least, that's the theory :-)

Regards,

Anthony Liguori
Avoid making system calls for out{b,w,l} instructions since it is not necessary
to sync GP registers.

Signed-off-by: Anthony Liguori <[EMAIL PROTECTED]>

diff -r 29119439ef33 user/kvmctl.c
--- a/user/kvmctl.c	Sat Feb 03 18:50:24 2007 -0600
+++ b/user/kvmctl.c	Sat Feb 03 19:07:24 2007 -0600
@@ -234,11 +234,14 @@ static int handle_io(kvm_context_t kvm, 
 	int first_time = 1;
 	int delta;
 	struct translation_cache tr;
+	int _in = (run->io.direction == KVM_EXIT_IO_IN);
 
 	translation_cache_init(&tr);
 
-	regs.vcpu = run->vcpu;
-	ioctl(kvm->fd, KVM_GET_REGS, &regs);
+	if (run->io.string || _in) {
+		regs.vcpu = run->vcpu;
+		ioctl(kvm->fd, KVM_GET_REGS, &regs);
+	}
 
 	delta = run->io.string_down ? -run->io.size : run->io.size;
 
@@ -246,9 +249,12 @@ static int handle_io(kvm_context_t kvm, 
 		void *value_addr;
 		int r;
 
-		if (!run->io.string)
-			value_addr = &regs.rax;
-		else {
+		if (!run->io.string) {
+			if (_in)
+				value_addr = &regs.rax;
+			else
+				value_addr = &run->io.value;
+		} else {
 			r = translate(kvm, run->vcpu, &tr, run->io.address, 
 				      &value_addr);
 			if (r) {
@@ -326,7 +332,8 @@ static int handle_io(kvm_context_t kvm, 
 		}
 	}
 
-	ioctl(kvm->fd, KVM_SET_REGS, &regs);
+	if (run->io.string || _in)
+		ioctl(kvm->fd, KVM_SET_REGS, &regs);
 	run->emulated = 1;
 	return 0;
 }
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