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[Xen-devel] Re: [PATCH 0/3][IA64] Accelerate IDE PIO on HVM/IA64

On Tue, 2007-02-27 at 18:34 +0900, Kouya SHIMURA wrote:
> Hi,
> This set of patches remarkably accelerates IDE PIO on HVM/IA64.
> I got a throughput of 2.11MB/sec in disk read performance.
> Without it, it was only 64kB/sec.
> I posted the prototype once. 
> http://lists.xensource.com/archives/html/xen-devel/2006-12/msg00077.html
> The basic idea is to add a buffering mechanism in a hypervisor.
> I know this approach is not sophisticated. But there is no other
> good way in IA64 which has no string instructions like x86's.

   This seems like a pretty good performance increase for only a single
page of domain overhead in Xen.  My main concern would be that we
maintain correctness.  It is writing out the dirty buffer before the
domain shuts down, right?  I'm guessing the flush probably does that.
Looks like a bug here though:

+static inline void
+buffered_pio_write(IDEState *s, uint32_t addr, int size)
+    struct pio_buffer *piobuf = piobuf_by_addr(addr);
+    uint32_t data_end;
+    if (!piobuf)
+        return;
+    buffered_pio_flush(piobuf);
+    data_end = s->data_end - s->data_ptr - size;
+    if (data_end <= 0)
+        data_end = 0;

   data_end is unsigned, so it will never be < 0.  Same problem on the
read func.  Thanks,


Alex Williamson                             HP Open Source & Linux Org.

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