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vmx: mask out bits 0:2 and 24:30 of pci CONFIG_ADDRESS on read

These bits are reserved, and by the specification,
must return zero on read.

This is also used by plan 9 for detecting config mode #1.

--- a/sys/src/cmd/vmx/pci.c

+++ b/sys/src/cmd/vmx/pci.c

@@ -237,7 +237,7 @@

 	switch(isin << 16 | port){

 	case 0x0cf8: cfgaddr = val; return 0;

-	case 0x10cf8: return cfgaddr;

+	case 0x10cf8: return cfgaddr & ~0x7f000003;

 	case 0xcfc: case 0xcfd: case 0xcfe: case 0xcff:

 		val <<= 8 * (port & 3);

 		mask = -1UL >> 32 - 8 * sz << 8 * (port & 3);