Re: [PATCH v2 2/2] kvm: add support for guest physical bits

[Tao Su <tao1.su@linux.intel.com>]

On Tue, Mar 05, 2024 at 11:52:33AM +0100, Gerd Hoffmann wrote:
> Query kvm for supported guest physical address bits, in cpuid
> function 80000008, eax[23:16].  Usually this is identical to host
> physical address bits.  With NPT or EPT being used this might be
> restricted to 48 (max 4-level paging address space size) even if
> the host cpu supports more physical address bits.
> 
> When set pass this to the guest, using cpuid too.  Guest firmware
> can use this to figure how big the usable guest physical address
> space is, so PCI bar mapping are actually reachable.

If this patch is applied, do you have plans to implement it in
OVMF/Seabios?

Thanks,
Tao

> 
> Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
> ---
>  target/i386/cpu.h     |  1 +
>  target/i386/cpu.c     |  1 +
>  target/i386/kvm/kvm.c | 17 +++++++++++++++++
>  3 files changed, 19 insertions(+)
> 
> diff --git a/target/i386/cpu.h b/target/i386/cpu.h
> index 952174bb6f52..d427218827f6 100644
> --- a/target/i386/cpu.h
> +++ b/target/i386/cpu.h
> @@ -2026,6 +2026,7 @@ struct ArchCPU {
>  
>      /* Number of physical address bits supported */
>      uint32_t phys_bits;
> +    uint32_t guest_phys_bits;
>  
>      /* in order to simplify APIC support, we leave this pointer to the
>         user */
> diff --git a/target/i386/cpu.c b/target/i386/cpu.c
> index 2666ef380891..1a6cfc75951e 100644
> --- a/target/i386/cpu.c
> +++ b/target/i386/cpu.c
> @@ -6570,6 +6570,7 @@ void cpu_x86_cpuid(CPUX86State *env, uint32_t index, uint32_t count,
>          if (env->features[FEAT_8000_0001_EDX] & CPUID_EXT2_LM) {
>              /* 64 bit processor */
>               *eax |= (cpu_x86_virtual_addr_width(env) << 8);
> +             *eax |= (cpu->guest_phys_bits << 16);
>          }
>          *ebx = env->features[FEAT_8000_0008_EBX];
>          if (cs->nr_cores * cs->nr_threads > 1) {
> diff --git a/target/i386/kvm/kvm.c b/target/i386/kvm/kvm.c
> index 7298822cb511..ce22dfcaa661 100644
> --- a/target/i386/kvm/kvm.c
> +++ b/target/i386/kvm/kvm.c
> @@ -238,6 +238,15 @@ static int kvm_get_tsc(CPUState *cs)
>      return 0;
>  }
>  
> +/* return cpuid fn 8000_0008 eax[23:16] aka GuestPhysBits */
> +static int kvm_get_guest_phys_bits(KVMState *s)
> +{
> +    uint32_t eax;
> +
> +    eax = kvm_arch_get_supported_cpuid(s, 0x80000008, 0, R_EAX);
> +    return (eax >> 16) & 0xff;
> +}
> +
>  static inline void do_kvm_synchronize_tsc(CPUState *cpu, run_on_cpu_data arg)
>  {
>      kvm_get_tsc(cpu);
> @@ -1730,6 +1739,7 @@ int kvm_arch_init_vcpu(CPUState *cs)
>      X86CPU *cpu = X86_CPU(cs);
>      CPUX86State *env = &cpu->env;
>      uint32_t limit, i, j, cpuid_i;
> +    uint32_t guest_phys_bits;
>      uint32_t unused;
>      struct kvm_cpuid_entry2 *c;
>      uint32_t signature[3];
> @@ -1765,6 +1775,13 @@ int kvm_arch_init_vcpu(CPUState *cs)
>  
>      env->apic_bus_freq = KVM_APIC_BUS_FREQUENCY;
>  
> +    guest_phys_bits = kvm_get_guest_phys_bits(cs->kvm_state);
> +    if (guest_phys_bits &&
> +        (cpu->guest_phys_bits == 0 ||
> +         cpu->guest_phys_bits > guest_phys_bits)) {
> +        cpu->guest_phys_bits = guest_phys_bits;
> +    }
> +
>      /*
>       * kvm_hyperv_expand_features() is called here for the second time in case
>       * KVM_CAP_SYS_HYPERV_CPUID is not supported. While we can't possibly handle
> -- 
> 2.44.0
> 
>