Re: [PATCH v3] x86: add etc/phys-bits fw_cfg file

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Sep 22, 2022 at 10:43:56AM +0200, Gerd Hoffmann wrote:
> In case phys bits are functional and can be used by the guest (aka
> host-phys-bits=on) add a fw_cfg file carrying the value.  This can
> be used by the guest firmware for address space configuration.
> 
> This is only enabled for 7.2+ machine types for live migration
> compatibility reasons.
> 
> Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

I'm curious why you decided to switch from a cpuid flag to fw cfg.  I
guess firmware reads fw cfg anyway. But would the guest kernel then need
to load a fw cfg driver very early to detect this, too?

> ---
>  hw/i386/fw_cfg.h     |  1 +
>  include/hw/i386/pc.h |  1 +
>  hw/i386/fw_cfg.c     | 12 ++++++++++++
>  hw/i386/pc.c         |  5 +++++
>  hw/i386/pc_piix.c    |  2 ++
>  hw/i386/pc_q35.c     |  2 ++
>  6 files changed, 23 insertions(+)
> 
> diff --git a/hw/i386/fw_cfg.h b/hw/i386/fw_cfg.h
> index 275f15c1c5e8..6ff198a6cb85 100644
> --- a/hw/i386/fw_cfg.h
> +++ b/hw/i386/fw_cfg.h
> @@ -26,5 +26,6 @@ FWCfgState *fw_cfg_arch_create(MachineState *ms,
>  void fw_cfg_build_smbios(MachineState *ms, FWCfgState *fw_cfg);
>  void fw_cfg_build_feature_control(MachineState *ms, FWCfgState *fw_cfg);
>  void fw_cfg_add_acpi_dsdt(Aml *scope, FWCfgState *fw_cfg);
> +void fw_cfg_phys_bits(FWCfgState *fw_cfg);
>  
>  #endif
> diff --git a/include/hw/i386/pc.h b/include/hw/i386/pc.h
> index c95333514ed3..bedef1ee13c1 100644
> --- a/include/hw/i386/pc.h
> +++ b/include/hw/i386/pc.h
> @@ -119,6 +119,7 @@ struct PCMachineClass {
>      bool enforce_aligned_dimm;
>      bool broken_reserved_end;
>      bool enforce_amd_1tb_hole;
> +    bool phys_bits_in_fw_cfg;
>  
>      /* generate legacy CPU hotplug AML */
>      bool legacy_cpu_hotplug;
> diff --git a/hw/i386/fw_cfg.c b/hw/i386/fw_cfg.c
> index a283785a8de4..6a1f18925725 100644
> --- a/hw/i386/fw_cfg.c
> +++ b/hw/i386/fw_cfg.c
> @@ -219,3 +219,15 @@ void fw_cfg_add_acpi_dsdt(Aml *scope, FWCfgState *fw_cfg)
>      aml_append(dev, aml_name_decl("_CRS", crs));
>      aml_append(scope, dev);
>  }
> +
> +void fw_cfg_phys_bits(FWCfgState *fw_cfg)
> +{
> +    X86CPU *cpu = X86_CPU(first_cpu);
> +    uint64_t phys_bits = cpu->phys_bits;
> +
> +    if (cpu->host_phys_bits) {
> +        fw_cfg_add_file(fw_cfg, "etc/phys-bits",
> +                        g_memdup2(&phys_bits, sizeof(phys_bits)),
> +                        sizeof(phys_bits));
> +    }
> +}

So, this allows a lot of flexibility, any phys_bits value at all can now
be used. Do you expect a use-case for such a flexible mechanism?  If
this ends up merely repeating CPUID at all times then we are just
creating confusion.
Could you add the motivation to the commit log pls?


> diff --git a/hw/i386/pc.c b/hw/i386/pc.c
> index 566accf7e60a..17ecc7fe4331 100644
> --- a/hw/i386/pc.c
> +++ b/hw/i386/pc.c
> @@ -744,6 +744,7 @@ void pc_machine_done(Notifier *notifier, void *data)
>  {
>      PCMachineState *pcms = container_of(notifier,
>                                          PCMachineState, machine_done);
> +    PCMachineClass *pcmc = PC_MACHINE_GET_CLASS(pcms);
>      X86MachineState *x86ms = X86_MACHINE(pcms);
>  
>      cxl_hook_up_pxb_registers(pcms->bus, &pcms->cxl_devices_state,
> @@ -764,6 +765,9 @@ void pc_machine_done(Notifier *notifier, void *data)
>          fw_cfg_build_feature_control(MACHINE(pcms), x86ms->fw_cfg);
>          /* update FW_CFG_NB_CPUS to account for -device added CPUs */
>          fw_cfg_modify_i16(x86ms->fw_cfg, FW_CFG_NB_CPUS, x86ms->boot_cpus);
> +        if (pcmc->phys_bits_in_fw_cfg) {
> +            fw_cfg_phys_bits(x86ms->fw_cfg);
> +        }
>      }
>  }
>  
> @@ -1907,6 +1911,7 @@ static void pc_machine_class_init(ObjectClass *oc, void *data)
>      pcmc->kvmclock_enabled = true;
>      pcmc->enforce_aligned_dimm = true;
>      pcmc->enforce_amd_1tb_hole = true;
> +    pcmc->phys_bits_in_fw_cfg = 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 8043a250adf3..c6a4dbd5c0b0 100644
> --- a/hw/i386/pc_piix.c
> +++ b/hw/i386/pc_piix.c
> @@ -447,9 +447,11 @@ DEFINE_I440FX_MACHINE(v7_2, "pc-i440fx-7.2", NULL,
>  
>  static void pc_i440fx_7_1_machine_options(MachineClass *m)
>  {
> +    PCMachineClass *pcmc = PC_MACHINE_CLASS(m);
>      pc_i440fx_7_2_machine_options(m);
>      m->alias = NULL;
>      m->is_default = false;
> +    pcmc->phys_bits_in_fw_cfg = false;
>      compat_props_add(m->compat_props, hw_compat_7_1, hw_compat_7_1_len);
>      compat_props_add(m->compat_props, pc_compat_7_1, pc_compat_7_1_len);
>  }
> diff --git a/hw/i386/pc_q35.c b/hw/i386/pc_q35.c
> index 53eda50e818c..c2b56daa1550 100644
> --- a/hw/i386/pc_q35.c
> +++ b/hw/i386/pc_q35.c
> @@ -384,8 +384,10 @@ DEFINE_Q35_MACHINE(v7_2, "pc-q35-7.2", NULL,
>  
>  static void pc_q35_7_1_machine_options(MachineClass *m)
>  {
> +    PCMachineClass *pcmc = PC_MACHINE_CLASS(m);
>      pc_q35_7_2_machine_options(m);
>      m->alias = NULL;
> +    pcmc->phys_bits_in_fw_cfg = false;
>      compat_props_add(m->compat_props, hw_compat_7_1, hw_compat_7_1_len);
>      compat_props_add(m->compat_props, pc_compat_7_1, pc_compat_7_1_len);
>  }
> -- 
> 2.37.3