Re: [PATCH 1/1] kvm: add support for guest physical bits

[Xiaoyao Li <xiaoyao.li@intel.com>]

On 3/1/2024 6:17 PM, Gerd Hoffmann wrote:
> query kvm for supported guest physical address bits using
> KVM_CAP_VM_GPA_BITS.  Expose the value to the guest via cpuid
> (leaf 0x80000008, eax, bits 16-23).
> 
> Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
> ---
>   target/i386/cpu.h     | 1 +
>   target/i386/cpu.c     | 1 +
>   target/i386/kvm/kvm.c | 8 ++++++++
>   3 files changed, 10 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);

I think you misunderstand this field.

If you expose this field to guest, it's the information for nested 
guest. i.e., the guest itself runs as a hypervisor will know its nested 
guest can have guest_phys_bits for physical addr.

>           }
>           *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 42970ab046fa..e06c9d66bb01 100644
> --- a/target/i386/kvm/kvm.c
> +++ b/target/i386/kvm/kvm.c
> @@ -1716,6 +1716,7 @@ 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];
> @@ -1751,6 +1752,13 @@ int kvm_arch_init_vcpu(CPUState *cs)
>   
>       env->apic_bus_freq = KVM_APIC_BUS_FREQUENCY;
>   
> +    guest_phys_bits = kvm_check_extension(cs->kvm_state, KVM_CAP_VM_GPA_BITS);
> +    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