On Fri, Mar 01, 2024, Gerd Hoffmann wrote:
> Add new guest_phys_bits field to kvm_caps, return the value to
> userspace when asked for KVM_CAP_VM_GPA_BITS capability.
>
> Initialize guest_phys_bits with boot_cpu_data.x86_phys_bits.
> Vendor modules (i.e. vmx and svm) can adjust this field in case
> additional restrictions apply, for example in case EPT has no
> support for 5-level paging.
>
> Signed-off-by: Gerd Hoffmann <kraxel@xxxxxxxxxx>
> ---
> arch/x86/kvm/x86.h | 2 ++
> arch/x86/kvm/x86.c | 5 +++++
> 2 files changed, 7 insertions(+)
>
> diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
> index 2f7e19166658..e03aec3527f8 100644
> --- a/arch/x86/kvm/x86.h
> +++ b/arch/x86/kvm/x86.h
> @@ -24,6 +24,8 @@ struct kvm_caps {
> bool has_bus_lock_exit;
> /* notify VM exit supported? */
> bool has_notify_vmexit;
> + /* usable guest phys bits */
> + u32 guest_phys_bits;
>
> u64 supported_mce_cap;
> u64 supported_xcr0;
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 48a61d283406..e270b9b708d1 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -4784,6 +4784,9 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
> if (kvm_is_vm_type_supported(KVM_X86_SW_PROTECTED_VM))
> r |= BIT(KVM_X86_SW_PROTECTED_VM);
> break;
> + case KVM_CAP_VM_GPA_BITS:
> + r = kvm_caps.guest_phys_bits;
This is not a fast path, just compute the effective guest.MAXPHYADDR on the fly
using tdp_root_level and max_tdp_level. But as pointed out and discussed in the
previous thread, adverising a guest.MAXPHYADDR that is smaller than host.MAXPHYADDR
simply doesn't work[*].
I thought the plan was to add a way for KVM to advertise the maximum *addressable*
GPA, and figure out a way to communicate that to the guest, e.g. so that firmware
doesn't try to use legal GPAs that the host cannot address.
Paolo, any update on this?
[*] https://lore.kernel.org/all/CALMp9eTutnTxCjQjs-nxP=XC345vTmJJODr+PcSOeaQpBW0Skw@xxxxxxxxxxxxxx
