iommu/vt-d: Serialize IOMMU GCMD register modifications

The VT-d spec requires (10.4.4 Global Command Register, GCMD_REG General
Description) that:

If multiple control fields in this register need to be modified, software
must serialize the modifications through multiple writes to this register.

However, in irq_remapping.c, modifications of IRE and CFI are done in one
write. We need to do two separate writes with STS checking after each. It
also checks the status register before writing command register to avoid
unnecessary register write.

Fixes: af8d102f ("x86/intel/irq_remapping: Clean up x2apic opt-out security warning mess")
Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Jacob Pan <jacob.jun.pan@linux.intel.com>
Cc: Kevin Tian <kevin.tian@intel.com>
Cc: Ashok Raj <ashok.raj@intel.com>
Link: https://lore.kernel.org/r/20200828000615.8281-1-baolu.lu@linux.intel.comSigned-off-by: Joerg Roedel <jroedel@suse.de>

drivers/iommu/intel/irq_remapping.c

@@ -508,12 +508,18 @@ static void iommu_enable_irq_remapping(struct intel_iommu *iommu)
 
 	/* Enable interrupt-remapping */
 	iommu->gcmd |= DMA_GCMD_IRE;
-	iommu->gcmd &= ~DMA_GCMD_CFI;  /* Block compatibility-format MSIs */
 	writel(iommu->gcmd, iommu->reg + DMAR_GCMD_REG);
-
 	IOMMU_WAIT_OP(iommu, DMAR_GSTS_REG,
 		      readl, (sts & DMA_GSTS_IRES), sts);
 
+	/* Block compatibility-format MSIs */
+	if (sts & DMA_GSTS_CFIS) {
+		iommu->gcmd &= ~DMA_GCMD_CFI;
+		writel(iommu->gcmd, iommu->reg + DMAR_GCMD_REG);
+		IOMMU_WAIT_OP(iommu, DMAR_GSTS_REG,
+			      readl, !(sts & DMA_GSTS_CFIS), sts);
+	}
+
 	/*
 	 * With CFI clear in the Global Command register, we should be
 	 * protected from dangerous (i.e. compatibility) interrupts