i386/pc: restrict AMD only enforcing of 1Tb hole to new machine type

The added enforcing is only relevant in the case of AMD where the
range right before the 1TB is restricted and cannot be DMA mapped
by the kernel consequently leading to IOMMU INVALID_DEVICE_REQUEST
or possibly other kinds of IOMMU events in the AMD IOMMU.

Although, there's a case where it may make sense to disable the
IOVA relocation/validation when migrating from a
non-amd-1tb-aware qemu to one that supports it.

Relocating RAM regions to after the 1Tb hole has consequences for
guest ABI because we are changing the memory mapping, so make
sure that only new machine enforce but not older ones.

Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
Acked-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Acked-by: Igor Mammedov <imammedo@redhat.com>
Message-Id: <20220719170014.27028-12-joao.m.martins@oracle.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

4 files changed, 7 insertions, 2 deletions

diff --git a/hw/i386/pc.c b/hw/i386/pc.c
index 1c5c9e17c6..7280c02ce3 100644
--- a/hw/i386/pc.c
+++ b/hw/i386/pc.c
@@ -951,9 +951,10 @@ void pc_memory_init(PCMachineState *pcms,
     /*
      * The HyperTransport range close to the 1T boundary is unique to AMD
      * hosts with IOMMUs enabled. Restrict the ram-above-4g relocation
-     * to above 1T to AMD vCPUs only.
+     * to above 1T to AMD vCPUs only. @enforce_amd_1tb_hole is only false in
+     * older machine types (<= 7.0) for compatibility purposes.
      */
-    if (IS_AMD_CPU(&cpu->env)) {
+    if (IS_AMD_CPU(&cpu->env) && pcmc->enforce_amd_1tb_hole) {
         /* Bail out if max possible address does not cross HT range */
         if (pc_max_used_gpa(pcms, pci_hole64_size) >= AMD_HT_START) {
             x86ms->above_4g_mem_start = AMD_ABOVE_1TB_START;
@@ -1902,6 +1903,7 @@ static void pc_machine_class_init(ObjectClass *oc, void *data)
     pcmc->has_reserved_memory = true;
     pcmc->kvmclock_enabled = true;
     pcmc->enforce_aligned_dimm = true;
+    pcmc->enforce_amd_1tb_hole = true;
     /* BIOS ACPI tables: 128K. Other BIOS datastructures: less than 4K reported
      * to be used at the moment, 32K should be enough for a while.  */
     pcmc->acpi_data_size = 0x20000 + 0x8000;
diff --git a/hw/i386/pc_piix.c b/hw/i386/pc_piix.c
index aa191d405a..a5c65c1c35 100644
--- a/hw/i386/pc_piix.c
+++ b/hw/i386/pc_piix.c
@@ -451,6 +451,7 @@ static void pc_i440fx_7_0_machine_options(MachineClass *m)
     m->alias = NULL;
     m->is_default = false;
     pcmc->legacy_no_rng_seed = true;
+    pcmc->enforce_amd_1tb_hole = false;
     compat_props_add(m->compat_props, hw_compat_7_0, hw_compat_7_0_len);
     compat_props_add(m->compat_props, pc_compat_7_0, pc_compat_7_0_len);
 }
diff --git a/hw/i386/pc_q35.c b/hw/i386/pc_q35.c
index 307910b33c..3a35193ff7 100644
--- a/hw/i386/pc_q35.c
+++ b/hw/i386/pc_q35.c
@@ -387,6 +387,7 @@ static void pc_q35_7_0_machine_options(MachineClass *m)
     pc_q35_7_1_machine_options(m);
     m->alias = NULL;
     pcmc->legacy_no_rng_seed = true;
+    pcmc->enforce_amd_1tb_hole = false;
     compat_props_add(m->compat_props, hw_compat_7_0, hw_compat_7_0_len);
     compat_props_add(m->compat_props, pc_compat_7_0, pc_compat_7_0_len);
 }
diff --git a/include/hw/i386/pc.h b/include/hw/i386/pc.h
index 01938fce4c..8435733bd6 100644
--- a/include/hw/i386/pc.h
+++ b/include/hw/i386/pc.h
@@ -118,6 +118,7 @@ struct PCMachineClass {
     bool has_reserved_memory;
     bool enforce_aligned_dimm;
     bool broken_reserved_end;
+    bool enforce_amd_1tb_hole;
 
     /* generate legacy CPU hotplug AML */
     bool legacy_cpu_hotplug;