iommu/vt-d: Serialize IOMMU GCMD register modifications

iommu/vt-d: Serialize IOMMU GCMD register modifications

BugLink: https://bugs.launchpad.net/bugs/1895880
[ Upstream commit 6e4e9ec65078093165463c13d4eb92b3e8d7b2e8 ]

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: af8d102f999a4 ("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.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: William Breathitt Gray <william.gray@canonical.com>
Signed-off-by: Kelsey Skunberg <kelsey.skunberg@canonical.com>