Subject: Re: [PATCH v13 23/35] KVM: x86: Add support for "protected VMs" that can utilize private memory

From: Paolo Bonzini <[email-masked]>
Date: Mon, 6 Nov 2023 12:03:03 +0100

On 11/6/23 12:00, Fuad Tabba wrote:
> Hi,
> 
> 
> On Fri, Oct 27, 2023 at 7:23 PM Sean Christopherson <[email-masked]> wrote:
>>
>> Add a new x86 VM type, KVM_X86_SW_PROTECTED_VM, to serve as a development
>> and testing vehicle for Confidential (CoCo) VMs, and potentially to even
>> become a "real" product in the distant future, e.g. a la pKVM.
>>
>> The private memory support in KVM x86 is aimed at AMD's SEV-SNP and
>> Intel's TDX, but those technologies are extremely complex (understatement),
>> difficult to debug, don't support running as nested guests, and require
>> hardware that's isn't universally accessible.  I.e. relying SEV-SNP or TDX
> 
> nit: "that isn't"
> 
> Reviewed-by: Fuad Tabba <[email-masked]>
> Tested-by: Fuad Tabba <[email-masked]>

Hi Fuad,

thanks for your reviews and tests of the gmem patches!  Can you please 
continue replying to v14?

Thanks,

Paolo

> Cheers,
> /fuad
> 
>> for maintaining guest private memory isn't a realistic option.
>>
>> At the very least, KVM_X86_SW_PROTECTED_VM will enable a variety of
>> selftests for guest_memfd and private memory support without requiring
>> unique hardware.
>>
>> Signed-off-by: Sean Christopherson <[email-masked]>
>> ---
>>   Documentation/virt/kvm/api.rst  | 32 ++++++++++++++++++++++++++++++++
>>   arch/x86/include/asm/kvm_host.h | 15 +++++++++------
>>   arch/x86/include/uapi/asm/kvm.h |  3 +++
>>   arch/x86/kvm/Kconfig            | 12 ++++++++++++
>>   arch/x86/kvm/mmu/mmu_internal.h |  1 +
>>   arch/x86/kvm/x86.c              | 16 +++++++++++++++-
>>   include/uapi/linux/kvm.h        |  1 +
>>   virt/kvm/Kconfig                |  5 +++++
>>   8 files changed, 78 insertions(+), 7 deletions(-)
>>
>> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
>> index 38dc1fda4f45..00029436ac5b 100644
>> --- a/Documentation/virt/kvm/api.rst
>> +++ b/Documentation/virt/kvm/api.rst
>> @@ -147,10 +147,29 @@ described as 'basic' will be available.
>>   The new VM has no virtual cpus and no memory.
>>   You probably want to use 0 as machine type.
>>
>> +X86:
>> +^^^^
>> +
>> +Supported X86 VM types can be queried via KVM_CAP_VM_TYPES.
>> +
>> +S390:
>> +^^^^^
>> +
>>   In order to create user controlled virtual machines on S390, check
>>   KVM_CAP_S390_UCONTROL and use the flag KVM_VM_S390_UCONTROL as
>>   privileged user (CAP_SYS_ADMIN).
>>
>> +MIPS:
>> +^^^^^
>> +
>> +To use hardware assisted virtualization on MIPS (VZ ASE) rather than
>> +the default trap & emulate implementation (which changes the virtual
>> +memory layout to fit in user mode), check KVM_CAP_MIPS_VZ and use the
>> +flag KVM_VM_MIPS_VZ.
>> +
>> +ARM64:
>> +^^^^^^
>> +
>>   On arm64, the physical address size for a VM (IPA Size limit) is limited
>>   to 40bits by default. The limit can be configured if the host supports the
>>   extension KVM_CAP_ARM_VM_IPA_SIZE. When supported, use
>> @@ -8650,6 +8669,19 @@ block sizes is exposed in KVM_CAP_ARM_SUPPORTED_BLOCK_SIZES as a
>>   64-bit bitmap (each bit describing a block size). The default value is
>>   0, to disable the eager page splitting.
>>
>> +8.41 KVM_CAP_VM_TYPES
>> +---------------------
>> +
>> +:Capability: KVM_CAP_MEMORY_ATTRIBUTES
>> +:Architectures: x86
>> +:Type: system ioctl
>> +
>> +This capability returns a bitmap of support VM types.  The 1-setting of bit @n
>> +means the VM type with value @n is supported.  Possible values of @n are::
>> +
>> +  #define KVM_X86_DEFAULT_VM   0
>> +  #define KVM_X86_SW_PROTECTED_VM      1
>> +
>>   9. Known KVM API problems
>>   =========================
>>
>> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
>> index f9e8d5642069..dff10051e9b6 100644
>> --- a/arch/x86/include/asm/kvm_host.h
>> +++ b/arch/x86/include/asm/kvm_host.h
>> @@ -1244,6 +1244,7 @@ enum kvm_apicv_inhibit {
>>   };
>>
>>   struct kvm_arch {
>> +       unsigned long vm_type;
>>          unsigned long n_used_mmu_pages;
>>          unsigned long n_requested_mmu_pages;
>>          unsigned long n_max_mmu_pages;
>> @@ -2077,6 +2078,12 @@ void kvm_mmu_new_pgd(struct kvm_vcpu *vcpu, gpa_t new_pgd);
>>   void kvm_configure_mmu(bool enable_tdp, int tdp_forced_root_level,
>>                         int tdp_max_root_level, int tdp_huge_page_level);
>>
>> +#ifdef CONFIG_KVM_PRIVATE_MEM
>> +#define kvm_arch_has_private_mem(kvm) ((kvm)->arch.vm_type != KVM_X86_DEFAULT_VM)
>> +#else
>> +#define kvm_arch_has_private_mem(kvm) false
>> +#endif
>> +
>>   static inline u16 kvm_read_ldt(void)
>>   {
>>          u16 ldt;
>> @@ -2125,14 +2132,10 @@ enum {
>>   #define HF_SMM_INSIDE_NMI_MASK (1 << 2)
>>
>>   # define KVM_MAX_NR_ADDRESS_SPACES     2
>> +/* SMM is currently unsupported for guests with private memory. */
>> +# define kvm_arch_nr_memslot_as_ids(kvm) (kvm_arch_has_private_mem(kvm) ? 1 : 2)
>>   # define kvm_arch_vcpu_memslots_id(vcpu) ((vcpu)->arch.hflags & HF_SMM_MASK ? 1 : 0)
>>   # define kvm_memslots_for_spte_role(kvm, role) __kvm_memslots(kvm, (role).smm)
>> -
>> -static inline int kvm_arch_nr_memslot_as_ids(struct kvm *kvm)
>> -{
>> -       return KVM_MAX_NR_ADDRESS_SPACES;
>> -}
>> -
>>   #else
>>   # define kvm_memslots_for_spte_role(kvm, role) __kvm_memslots(kvm, 0)
>>   #endif
>> diff --git a/arch/x86/include/uapi/asm/kvm.h b/arch/x86/include/uapi/asm/kvm.h
>> index 1a6a1f987949..a448d0964fc0 100644
>> --- a/arch/x86/include/uapi/asm/kvm.h
>> +++ b/arch/x86/include/uapi/asm/kvm.h
>> @@ -562,4 +562,7 @@ struct kvm_pmu_event_filter {
>>   /* x86-specific KVM_EXIT_HYPERCALL flags. */
>>   #define KVM_EXIT_HYPERCALL_LONG_MODE   BIT(0)
>>
>> +#define KVM_X86_DEFAULT_VM     0
>> +#define KVM_X86_SW_PROTECTED_VM        1
>> +
>>   #endif /* _ASM_X86_KVM_H */
>> diff --git a/arch/x86/kvm/Kconfig b/arch/x86/kvm/Kconfig
>> index 091b74599c22..8452ed0228cb 100644
>> --- a/arch/x86/kvm/Kconfig
>> +++ b/arch/x86/kvm/Kconfig
>> @@ -77,6 +77,18 @@ config KVM_WERROR
>>
>>            If in doubt, say "N".
>>
>> +config KVM_SW_PROTECTED_VM
>> +       bool "Enable support for KVM software-protected VMs"
>> +       depends on EXPERT
>> +       depends on X86_64
>> +       select KVM_GENERIC_PRIVATE_MEM
>> +       help
>> +         Enable support for KVM software-protected VMs.  Currently "protected"
>> +         means the VM can be backed with memory provided by
>> +         KVM_CREATE_GUEST_MEMFD.
>> +
>> +         If unsure, say "N".
>> +
>>   config KVM_INTEL
>>          tristate "KVM for Intel (and compatible) processors support"
>>          depends on KVM && IA32_FEAT_CTL
>> diff --git a/arch/x86/kvm/mmu/mmu_internal.h b/arch/x86/kvm/mmu/mmu_internal.h
>> index 86c7cb692786..b66a7d47e0e4 100644
>> --- a/arch/x86/kvm/mmu/mmu_internal.h
>> +++ b/arch/x86/kvm/mmu/mmu_internal.h
>> @@ -297,6 +297,7 @@ static inline int kvm_mmu_do_page_fault(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
>>                  .max_level = KVM_MAX_HUGEPAGE_LEVEL,
>>                  .req_level = PG_LEVEL_4K,
>>                  .goal_level = PG_LEVEL_4K,
>> +               .is_private = kvm_mem_is_private(vcpu->kvm, cr2_or_gpa >> PAGE_SHIFT),
>>          };
>>          int r;
>>
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index c4d17727b199..e3eb608b6692 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -4441,6 +4441,13 @@ static int kvm_ioctl_get_supported_hv_cpuid(struct kvm_vcpu *vcpu,
>>          return 0;
>>   }
>>
>> +static bool kvm_is_vm_type_supported(unsigned long type)
>> +{
>> +       return type == KVM_X86_DEFAULT_VM ||
>> +              (type == KVM_X86_SW_PROTECTED_VM &&
>> +               IS_ENABLED(CONFIG_KVM_SW_PROTECTED_VM) && tdp_enabled);
>> +}
>> +
>>   int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
>>   {
>>          int r = 0;
>> @@ -4632,6 +4639,11 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
>>          case KVM_CAP_X86_NOTIFY_VMEXIT:
>>                  r = kvm_caps.has_notify_vmexit;
>>                  break;
>> +       case KVM_CAP_VM_TYPES:
>> +               r = BIT(KVM_X86_DEFAULT_VM);
>> +               if (kvm_is_vm_type_supported(KVM_X86_SW_PROTECTED_VM))
>> +                       r |= BIT(KVM_X86_SW_PROTECTED_VM);
>> +               break;
>>          default:
>>                  break;
>>          }
>> @@ -12314,9 +12326,11 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)
>>          int ret;
>>          unsigned long flags;
>>
>> -       if (type)
>> +       if (!kvm_is_vm_type_supported(type))
>>                  return -EINVAL;
>>
>> +       kvm->arch.vm_type = type;
>> +
>>          ret = kvm_page_track_init(kvm);
>>          if (ret)
>>                  goto out;
>> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
>> index 29e9eb51dec9..5b5820d19e71 100644
>> --- a/include/uapi/linux/kvm.h
>> +++ b/include/uapi/linux/kvm.h
>> @@ -1218,6 +1218,7 @@ struct kvm_ppc_resize_hpt {
>>   #define KVM_CAP_MEMORY_FAULT_INFO 231
>>   #define KVM_CAP_MEMORY_ATTRIBUTES 232
>>   #define KVM_CAP_GUEST_MEMFD 233
>> +#define KVM_CAP_VM_TYPES 234
>>
>>   #ifdef KVM_CAP_IRQ_ROUTING
>>
>> diff --git a/virt/kvm/Kconfig b/virt/kvm/Kconfig
>> index 08afef022db9..2c964586aa14 100644
>> --- a/virt/kvm/Kconfig
>> +++ b/virt/kvm/Kconfig
>> @@ -104,3 +104,8 @@ config KVM_GENERIC_MEMORY_ATTRIBUTES
>>   config KVM_PRIVATE_MEM
>>          select XARRAY_MULTI
>>          bool
>> +
>> +config KVM_GENERIC_PRIVATE_MEM
>> +       select KVM_GENERIC_MEMORY_ATTRIBUTES
>> +       select KVM_PRIVATE_MEM
>> +       bool
>> --
>> 2.42.0.820.g83a721a137-goog
>>
>