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

[Paolo Bonzini]

On Tue, Mar 5, 2024 at 11:52 AM Gerd Hoffmann <kraxel@redhat.com> 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.
>
> 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
> +    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;
> +    }

Like Xiaoyao mentioned, the right place for this is kvm_cpu_realizefn,
after host_cpu_realizefn returns. It should also be conditional on
cpu->host_phys_bits. It also makes sense to:

- make kvm_get_guest_phys_bits() return bits 7:0 if bits 23:16 are zero

- here, set cpu->guest_phys_bits only if it is not equal to
cpu->phys_bits (this undoes the previous suggestion, but I think it's
cleaner)

- add a property in x86_cpu_properties[] to allow configuration with TCG.

Paolo

> +
>      /*
>       * 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
>