pci-test is a device used for testing low level IO 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. 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 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. 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.