Re: [PATCH v8 5/8] KVM: Register/unregister the guest private memory regions

[Chao Peng <chao.p.peng@linux.intel.com>]

On Tue, Oct 11, 2022 at 10:48:58AM +0100, Fuad Tabba wrote:
> Hi,
> 
> On Thu, Sep 15, 2022 at 3:38 PM Chao Peng <chao.p.peng@linux.intel.com> wrote:
> >
> > If CONFIG_HAVE_KVM_PRIVATE_MEM=y, userspace can register/unregister the
> > guest private memory regions through KVM_MEMORY_ENCRYPT_{UN,}REG_REGION
> > ioctls. The patch reuses existing SEV ioctl number but differs that the
> > address in the region for KVM_PRIVATE_MEM case is gpa while for SEV case
> > it's hva. Which usages should the ioctls go is determined by the newly
> > added kvm_arch_has_private_mem(). Architecture which supports
> > KVM_PRIVATE_MEM should override this function.
> >
> > The current implementation defaults all memory to private. The shared
> > memory regions are stored in a xarray variable for memory efficiency and
> > zapping existing memory mappings is also a side effect of these two
> > ioctls when defined.
> >
> > Signed-off-by: Chao Peng <chao.p.peng@linux.intel.com>
> > ---
> >  Documentation/virt/kvm/api.rst  | 17 ++++++--
> >  arch/x86/include/asm/kvm_host.h |  1 +
> >  arch/x86/kvm/mmu.h              |  2 -
> >  include/linux/kvm_host.h        | 13 ++++++
> >  virt/kvm/kvm_main.c             | 73 +++++++++++++++++++++++++++++++++
> >  5 files changed, 100 insertions(+), 6 deletions(-)
> >
> > diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> > index 1a6c003b2a0b..c0f800d04ffc 100644
> > --- a/Documentation/virt/kvm/api.rst
> > +++ b/Documentation/virt/kvm/api.rst
> > @@ -4715,10 +4715,19 @@ Documentation/virt/kvm/x86/amd-memory-encryption.rst.
> >  This ioctl can be used to register a guest memory region which may
> >  contain encrypted data (e.g. guest RAM, SMRAM etc).
> >
> > -It is used in the SEV-enabled guest. When encryption is enabled, a guest
> > -memory region may contain encrypted data. The SEV memory encryption
> > -engine uses a tweak such that two identical plaintext pages, each at
> > -different locations will have differing ciphertexts. So swapping or
> > +Currently this ioctl supports registering memory regions for two usages:
> > +private memory and SEV-encrypted memory.
> > +
> > +When private memory is enabled, this ioctl is used to register guest private
> > +memory region and the addr/size of kvm_enc_region represents guest physical
> > +address (GPA). In this usage, this ioctl zaps the existing guest memory
> > +mappings in KVM that fallen into the region.
> > +
> > +When SEV-encrypted memory is enabled, this ioctl is used to register guest
> > +memory region which may contain encrypted data for a SEV-enabled guest. The
> > +addr/size of kvm_enc_region represents userspace address (HVA). The SEV
> > +memory encryption engine uses a tweak such that two identical plaintext pages,
> > +each at different locations will have differing ciphertexts. So swapping or
> >  moving ciphertext of those pages will not result in plaintext being
> >  swapped. So relocating (or migrating) physical backing pages for the SEV
> >  guest will require some additional steps.
> > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> > index 2c96c43c313a..cfad6ba1a70a 100644
> > --- a/arch/x86/include/asm/kvm_host.h
> > +++ b/arch/x86/include/asm/kvm_host.h
> > @@ -37,6 +37,7 @@
> >  #include <asm/hyperv-tlfs.h>
> >
> >  #define __KVM_HAVE_ARCH_VCPU_DEBUGFS
> > +#define __KVM_HAVE_ZAP_GFN_RANGE
> >
> >  #define KVM_MAX_VCPUS 1024
> >
> > diff --git a/arch/x86/kvm/mmu.h b/arch/x86/kvm/mmu.h
> > index 6bdaacb6faa0..c94b620bf94b 100644
> > --- a/arch/x86/kvm/mmu.h
> > +++ b/arch/x86/kvm/mmu.h
> > @@ -211,8 +211,6 @@ static inline u8 permission_fault(struct kvm_vcpu *vcpu, struct kvm_mmu *mmu,
> >         return -(u32)fault & errcode;
> >  }
> >
> > -void kvm_zap_gfn_range(struct kvm *kvm, gfn_t gfn_start, gfn_t gfn_end);
> > -
> >  int kvm_arch_write_log_dirty(struct kvm_vcpu *vcpu);
> >
> >  int kvm_mmu_post_init_vm(struct kvm *kvm);
> > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> > index 2125b50f6345..d65690cae80b 100644
> > --- a/include/linux/kvm_host.h
> > +++ b/include/linux/kvm_host.h
> > @@ -260,6 +260,15 @@ bool kvm_test_age_gfn(struct kvm *kvm, struct kvm_gfn_range *range);
> >  bool kvm_set_spte_gfn(struct kvm *kvm, struct kvm_gfn_range *range);
> >  #endif
> >
> > +#ifdef __KVM_HAVE_ZAP_GFN_RANGE
> > +void kvm_zap_gfn_range(struct kvm *kvm, gfn_t gfn_start, gfn_t gfn_end);
> > +#else
> > +static inline void kvm_zap_gfn_range(struct kvm *kvm, gfn_t gfn_start
> > +                                                     gfn_t gfn_end)
> > +{
> > +}
> > +#endif
> > +
> >  enum {
> >         OUTSIDE_GUEST_MODE,
> >         IN_GUEST_MODE,
> > @@ -795,6 +804,9 @@ struct kvm {
> >         struct notifier_block pm_notifier;
> >  #endif
> >         char stats_id[KVM_STATS_NAME_SIZE];
> > +#ifdef CONFIG_HAVE_KVM_PRIVATE_MEM
> > +       struct xarray mem_attr_array;
> > +#endif
> >  };
> >
> >  #define kvm_err(fmt, ...) \
> > @@ -1454,6 +1466,7 @@ bool kvm_arch_dy_has_pending_interrupt(struct kvm_vcpu *vcpu);
> >  int kvm_arch_post_init_vm(struct kvm *kvm);
> >  void kvm_arch_pre_destroy_vm(struct kvm *kvm);
> >  int kvm_arch_create_vm_debugfs(struct kvm *kvm);
> > +bool kvm_arch_has_private_mem(struct kvm *kvm);
> >
> >  #ifndef __KVM_HAVE_ARCH_VM_ALLOC
> >  /*
> > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > index fa9dd2d2c001..de5cce8c82c7 100644
> > --- a/virt/kvm/kvm_main.c
> > +++ b/virt/kvm/kvm_main.c
> > @@ -937,6 +937,47 @@ static int kvm_init_mmu_notifier(struct kvm *kvm)
> >
> >  #endif /* CONFIG_MMU_NOTIFIER && KVM_ARCH_WANT_MMU_NOTIFIER */
> >
> > +#ifdef CONFIG_HAVE_KVM_PRIVATE_MEM
> > +#define KVM_MEM_ATTR_SHARED    0x0001
> > +static int kvm_vm_ioctl_set_mem_attr(struct kvm *kvm, gpa_t gpa, gpa_t size,
> > +                                    bool is_private)
> > +{
> 
> I wonder if this ioctl should be implemented as an arch-specific
> ioctl. In this patch it performs some actions that pKVM might not need
> or might want to do differently.

I think it's doable. We can provide the mem_attr_array kind thing in
common code and let arch code decide to use it or not. Currently
mem_attr_array is defined in the struct kvm, if those bytes are
unnecessary for pKVM it can even be moved to arch definition, but that
also loses the potential code sharing for confidential usages in other
non-architectures, e.g. if ARM also supports such usage. Or it can be
provided through a different CONFIG_ instead of
CONFIG_HAVE_KVM_PRIVATE_MEM.

Thanks,
Chao
> 
> pKVM tracks the sharing status in the stage-2 page table's software
> bits, so it can avoid the overhead of using mem_attr_array.
> 
> Also, this ioctl calls kvm_zap_gfn_range(), as does the invalidation
> notifier (introduced in patch 8). For pKVM, the kind of zapping (or
> the information conveyed to the hypervisor) might need to be different
> depending on the cause; whether it's invalidation or change of sharing
> status.

> 
> Thanks,
> /fuad
> 
> 
> > +       gfn_t start, end;
> > +       unsigned long index;
> > +       void *entry;
> > +       int r;
> > +
> > +       if (size == 0 || gpa + size < gpa)
> > +               return -EINVAL;
> > +       if (gpa & (PAGE_SIZE - 1) || size & (PAGE_SIZE - 1))
> > +               return -EINVAL;
> > +
> > +       start = gpa >> PAGE_SHIFT;
> > +       end = (gpa + size - 1 + PAGE_SIZE) >> PAGE_SHIFT;
> > +
> > +       /*
> > +        * Guest memory defaults to private, kvm->mem_attr_array only stores
> > +        * shared memory.
> > +        */
> > +       entry = is_private ? NULL : xa_mk_value(KVM_MEM_ATTR_SHARED);
> > +
> > +       for (index = start; index < end; index++) {
> > +               r = xa_err(xa_store(&kvm->mem_attr_array, index, entry,
> > +                                   GFP_KERNEL_ACCOUNT));
> > +               if (r)
> > +                       goto err;
> > +       }
> > +
> > +       kvm_zap_gfn_range(kvm, start, end);
> > +
> > +       return r;
> > +err:
> > +       for (; index > start; index--)
> > +               xa_erase(&kvm->mem_attr_array, index);
> > +       return r;
> > +}
> > +#endif /* CONFIG_HAVE_KVM_PRIVATE_MEM */
> > +
> >  #ifdef CONFIG_HAVE_KVM_PM_NOTIFIER
> >  static int kvm_pm_notifier_call(struct notifier_block *bl,
> >                                 unsigned long state,
> > @@ -1165,6 +1206,9 @@ static struct kvm *kvm_create_vm(unsigned long type, const char *fdname)
> >         spin_lock_init(&kvm->mn_invalidate_lock);
> >         rcuwait_init(&kvm->mn_memslots_update_rcuwait);
> >         xa_init(&kvm->vcpu_array);
> > +#ifdef CONFIG_HAVE_KVM_PRIVATE_MEM
> > +       xa_init(&kvm->mem_attr_array);
> > +#endif
> >
> >         INIT_LIST_HEAD(&kvm->gpc_list);
> >         spin_lock_init(&kvm->gpc_lock);
> > @@ -1338,6 +1382,9 @@ static void kvm_destroy_vm(struct kvm *kvm)
> >                 kvm_free_memslots(kvm, &kvm->__memslots[i][0]);
> >                 kvm_free_memslots(kvm, &kvm->__memslots[i][1]);
> >         }
> > +#ifdef CONFIG_HAVE_KVM_PRIVATE_MEM
> > +       xa_destroy(&kvm->mem_attr_array);
> > +#endif
> >         cleanup_srcu_struct(&kvm->irq_srcu);
> >         cleanup_srcu_struct(&kvm->srcu);
> >         kvm_arch_free_vm(kvm);
> > @@ -1541,6 +1588,11 @@ static void kvm_replace_memslot(struct kvm *kvm,
> >         }
> >  }
> >
> > +bool __weak kvm_arch_has_private_mem(struct kvm *kvm)
> > +{
> > +       return false;
> > +}
> > +
> >  static int check_memory_region_flags(const struct kvm_user_mem_region *mem)
> >  {
> >         u32 valid_flags = KVM_MEM_LOG_DIRTY_PAGES;
> > @@ -4703,6 +4755,24 @@ static long kvm_vm_ioctl(struct file *filp,
> >                 r = kvm_vm_ioctl_set_memory_region(kvm, &mem);
> >                 break;
> >         }
> > +#ifdef CONFIG_HAVE_KVM_PRIVATE_MEM
> > +       case KVM_MEMORY_ENCRYPT_REG_REGION:
> > +       case KVM_MEMORY_ENCRYPT_UNREG_REGION: {
> > +               struct kvm_enc_region region;
> > +               bool set = ioctl == KVM_MEMORY_ENCRYPT_REG_REGION;
> > +
> > +               if (!kvm_arch_has_private_mem(kvm))
> > +                       goto arch_vm_ioctl;
> > +
> > +               r = -EFAULT;
> > +               if (copy_from_user(&region, argp, sizeof(region)))
> > +                       goto out;
> > +
> > +               r = kvm_vm_ioctl_set_mem_attr(kvm, region.addr,
> > +                                             region.size, set);
> > +               break;
> > +       }
> > +#endif
> >         case KVM_GET_DIRTY_LOG: {
> >                 struct kvm_dirty_log log;
> >
> > @@ -4856,6 +4926,9 @@ static long kvm_vm_ioctl(struct file *filp,
> >                 r = kvm_vm_ioctl_get_stats_fd(kvm);
> >                 break;
> >         default:
> > +#ifdef CONFIG_HAVE_KVM_PRIVATE_MEM
> > +arch_vm_ioctl:
> > +#endif
> >                 r = kvm_arch_vm_ioctl(filp, ioctl, arg);
> >         }
> >  out:
> > --
> > 2.25.1
> >