iommu/amd: Restore IRTE.RemapEn bit for amd_iommu_activate_guest_mode

Commit e52d58d54a ("iommu/amd: Use cmpxchg_double() when updating
128-bit IRTE") removed an assumption that modify_irte_ga always set
the valid bit, which requires the callers to set the appropriate value
for the struct irte_ga.valid bit before calling the function.

Similar to the commit 26e495f341 ("iommu/amd: Restore IRTE.RemapEn
bit after programming IRTE"), which is for the function
amd_iommu_deactivate_guest_mode().

The same change is also needed for the amd_iommu_activate_guest_mode().
Otherwise, this could trigger IO_PAGE_FAULT for the VFIO based VMs with
AVIC enabled.

Fixes: e52d58d54a ("iommu/amd: Use cmpxchg_double() when updating 128-bit IRTE")
Reported-by: Maxim Levitsky <mlevitsk@redhat.com>
Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
Tested-by: Maxim Levitsky <mlevitsk@redhat.com>
Reviewed-by: Joao Martins <joao.m.martins@oracle.com>
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Cc: Joao Martins <joao.m.martins@oracle.com>
Link: https://lore.kernel.org/r/20200916111720.43913-1-suravee.suthikulpanit@amd.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>

drivers/iommu/amd/iommu.c

@@ -3840,14 +3840,18 @@ int amd_iommu_activate_guest_mode(void *data)
 {
 	struct amd_ir_data *ir_data = (struct amd_ir_data *)data;
 	struct irte_ga *entry = (struct irte_ga *) ir_data->entry;
+	u64 valid;
 
 	if (!AMD_IOMMU_GUEST_IR_VAPIC(amd_iommu_guest_ir) ||
 	    !entry || entry->lo.fields_vapic.guest_mode)
 		return 0;
 
+	valid = entry->lo.fields_vapic.valid;
+
 	entry->lo.val = 0;
 	entry->hi.val = 0;
 
+	entry->lo.fields_vapic.valid       = valid;
 	entry->lo.fields_vapic.guest_mode  = 1;
 	entry->lo.fields_vapic.ga_log_intr = 1;
 	entry->hi.fields.ga_root_ptr       = ir_data->ga_root_ptr;