Re: [PATCH v5 5/5] i386/pc: restrict AMD only enforcing of valid IOVAs to new machine type

[Igor Mammedov <imammedo@redhat.com>]

On Fri, 20 May 2022 11:45:32 +0100
Joao Martins <joao.m.martins@oracle.com> wrote:

> 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-valid-IOVA-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 ons.

is old machine with so much ram going to work and not explode
even without iommu?

> Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
> ---
>  hw/i386/pc.c         | 7 +++++--
>  hw/i386/pc_piix.c    | 2 ++
>  hw/i386/pc_q35.c     | 2 ++
>  include/hw/i386/pc.h | 1 +
>  4 files changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/hw/i386/pc.c b/hw/i386/pc.c
> index 652ae8ff9ccf..62f9af91f19f 100644
> --- a/hw/i386/pc.c
> +++ b/hw/i386/pc.c
> @@ -862,6 +862,7 @@ static hwaddr x86_max_phys_addr(PCMachineState *pcms,
>  static void x86_update_above_4g_mem_start(PCMachineState *pcms,
>                                            uint64_t pci_hole64_size)
>  {
> +    PCMachineClass *pcmc = PC_MACHINE_GET_CLASS(pcms);
>      X86MachineState *x86ms = X86_MACHINE(pcms);
>      CPUX86State *env = &X86_CPU(first_cpu)->env;
>      hwaddr start = x86ms->above_4g_mem_start;
> @@ -870,9 +871,10 @@ static void x86_update_above_4g_mem_start(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_valid_iova is only false in
> +     * older machine types (<= 7.0) for compatibility purposes.
>       */
> -    if (!IS_AMD_CPU(env)) {
> +    if (!IS_AMD_CPU(env) || !pcmc->enforce_valid_iova) {
>          return;
>      }
>  
> @@ -1881,6 +1883,7 @@ 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_valid_iova = 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 57bb5b8f2aea..74176a210d56 100644
> --- a/hw/i386/pc_piix.c
> +++ b/hw/i386/pc_piix.c
> @@ -437,9 +437,11 @@ DEFINE_I440FX_MACHINE(v7_1, "pc-i440fx-7.1", NULL,
>  
>  static void pc_i440fx_7_0_machine_options(MachineClass *m)
>  {
> +    PCMachineClass *pcmc = PC_MACHINE_CLASS(m);
>      pc_i440fx_7_1_machine_options(m);
>      m->alias = NULL;
>      m->is_default = false;
> +    pcmc->enforce_valid_iova = 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 4d5c2fbd976b..bc38a6ba4c67 100644
> --- a/hw/i386/pc_q35.c
> +++ b/hw/i386/pc_q35.c
> @@ -381,8 +381,10 @@ DEFINE_Q35_MACHINE(v7_1, "pc-q35-7.1", NULL,
>  
>  static void pc_q35_7_0_machine_options(MachineClass *m)
>  {
> +    PCMachineClass *pcmc = PC_MACHINE_CLASS(m);
>      pc_q35_7_1_machine_options(m);
>      m->alias = NULL;
> +    pcmc->enforce_valid_iova = 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 9c847faea2f8..22119131eca7 100644
> --- a/include/hw/i386/pc.h
> +++ b/include/hw/i386/pc.h
> @@ -117,6 +117,7 @@ struct PCMachineClass {
>      bool has_reserved_memory;
>      bool enforce_aligned_dimm;
>      bool broken_reserved_end;
> +    bool enforce_valid_iova;
>  
>      /* generate legacy CPU hotplug AML */
>      bool legacy_cpu_hotplug;