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diff --git a/docs/specs/pci-testdev.txt b/docs/specs/pci-testdev.txt
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+pci-test is a device used for testing low level IO
+
+device implements up to two BARs: BAR0 and BAR1.
+Each BAR can be memory or IO. Guests must detect
+BAR type and act accordingly.
+
+Each BAR size is up to 4K bytes.
+Each BAR starts with the following header:
+
+typedef struct PCITestDevHdr {
+    uint8_t test;  <- write-only, starts a given test number
+    uint8_t width_type; <- read-only, type and width of access for a given test.
+                           1,2,4 for byte,word or long write.
+                           any other value if test not supported on this BAR
+    uint8_t pad0[2];
+    uint32_t offset; <- read-only, offset in this BAR for a given test
+    uint32_t data;    <- read-only, data to use for a given test
+    uint32_t count;  <- for debugging. number of writes detected.
+    uint8_t name[]; <- for debugging. 0-terminated ASCII string.
+} PCITestDevHdr;
+
+All registers are little endian.
+
+device is expected to always implement tests 0 to N on each BAR, and to add new
+tests with higher numbers.  In this way a guest can scan test numbers until it
+detects an access type that it does not support on this BAR, then stop.