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-rw-r--r--docs/specs/pci-testdev.txt15
1 files changed, 10 insertions, 5 deletions
diff --git a/docs/specs/pci-testdev.txt b/docs/specs/pci-testdev.txt
index 128ae222ef..4280a1e73c 100644
--- a/docs/specs/pci-testdev.txt
+++ b/docs/specs/pci-testdev.txt
@@ -1,11 +1,11 @@
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.
+device implements up to three BARs: BAR0, BAR1 and BAR2.
+Each of BAR 0+1 can be memory or IO. Guests must detect
+BAR types and act accordingly.
-Each BAR size is up to 4K bytes.
-Each BAR starts with the following header:
+BAR 0+1 size is up to 4K bytes each.
+BAR 0+1 starts with the following header:
typedef struct PCITestDevHdr {
uint8_t test; <- write-only, starts a given test number
@@ -24,3 +24,8 @@ 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.
+
+BAR2 is a 64bit memory bar, without backing storage. It is disabled
+by default and can be enabled using the membar=<size> property. This
+can be used to test whether guests handle pci bars of a specific
+(possibly quite large) size correctly.