diff --git a/sys/src/9/pc/mp.c b/sys/src/9/pc/mp.c
index b74fa1250..f3ca3067f 100644
--- a/sys/src/9/pc/mp.c
+++ b/sys/src/9/pc/mp.c
@@ -883,13 +883,14 @@ mpintrenablex(Vctl* v, int tbdf)
 enum {
 	MSICtrl = 0x02, /* message control register (16 bit) */
 	MSIAddr = 0x04, /* message address register (64 bit) */
-	MSIData = 0x0C, /* message data register (16 bit) */
+	MSIData32 = 0x08, /* message data register for 32 bit MSI (16 bit) */
+	MSIData64 = 0x0C, /* message data register for 64 bit MSI (16 bit) */
 };
 
 static int
 msiintrenable(Vctl *v)
 {
-	int tbdf, vno, cap, cpu;
+	int tbdf, vno, cap, cpu, ok64;
 	Pcidev *pci;
 
 	if(getconf("*msi") == nil)
@@ -913,9 +914,10 @@ msiintrenable(Vctl *v)
 	
 	vno = allocvector();
 	cpu = mpintrcpu();
+	ok64 = (pcicfgr16(pci, cap + MSICtrl) & (1<<7)) != 0;
 	pcicfgw32(pci, cap + MSIAddr, (0xFEE << 20) | (cpu << 12));
-	pcicfgw32(pci, cap + MSIAddr + 4, 0);
-	pcicfgw16(pci, cap + MSIData, vno | (1<<14));
+	if(ok64) pcicfgw32(pci, cap + MSIAddr + 4, 0);
+	pcicfgw16(pci, cap + ok64 ? MSIData64 : MSIData32, vno | (1<<14));
 	pcicfgw16(pci, cap + MSICtrl, 1);
 	print("msiintrenable: success with tbdf %.8x, vector %d, cpu %d\n", tbdf, vno, cpu);
 	v->isr = lapicisr;