Re: [PATCH v4] x86: add etc/phys-bits fw_cfg file

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

On Thu, Sep 22, 2022 at 12:24:09PM +0100, Daniel P. Berrangé wrote:
> On Thu, Sep 22, 2022 at 12:14:54PM +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.
> > 
> > The value in the etc/phys-bits fw_cfg file should be identical to
> > the phys bits value published via cpuid leaf 0x80000008.
> > 
> > This is only enabled for 7.2+ machine types for live migration
> > compatibility reasons.
> 
> Is this going to have any implications for what mgmt apps must
> take into account when selecting valid migration target hosts ?

I don't think this does anything by itself. It just tells the firmware
which value to use, since historically it ignored CPUID. I am still
debating with myself whether a boolean would be better.  Would
appreciate KVM maintainer's take on this. But in any case guests already
sometimes use CPUID (e.g. just grep for cpuid_maxphyaddr).
This value is just for firmware use.


> Historically, apps have tended to ignore any checks for phys
> bits between src/dst migration hosts and hoped for the best.
> 
> Will this new behaviour introduce / change any failure scenarios
> where the target host has fewer phys bits than the src host, that
> mgmt apps need to be made aware of ?
> 
> > 
> > Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
> > Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
> > ---
> >  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));
> > +    }
> > +}
> > 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
> > 
> > 
> 
> With regards,
> Daniel
> -- 
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