On Sun, Nov 5, 2023 at 4:32 PM Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote: > > From: Chao Peng <chao.p.peng@xxxxxxxxxxxxxxx> > > Add a new KVM exit type to allow userspace to handle memory faults that > KVM cannot resolve, but that userspace *may* be able to handle (without > terminating the guest). > > KVM will initially use KVM_EXIT_MEMORY_FAULT to report implicit > conversions between private and shared memory. With guest private memory, > there will be two kind of memory conversions: > > - explicit conversion: happens when the guest explicitly calls into KVM > to map a range (as private or shared) > > - implicit conversion: happens when the guest attempts to access a gfn > that is configured in the "wrong" state (private vs. shared) > > On x86 (first architecture to support guest private memory), explicit > conversions will be reported via KVM_EXIT_HYPERCALL+KVM_HC_MAP_GPA_RANGE, > but reporting KVM_EXIT_HYPERCALL for implicit conversions is undesriable > as there is (obviously) no hypercall, and there is no guarantee that the > guest actually intends to convert between private and shared, i.e. what > KVM thinks is an implicit conversion "request" could actually be the > result of a guest code bug. > > KVM_EXIT_MEMORY_FAULT will be used to report memory faults that appear to > be implicit conversions. > > Note! To allow for future possibilities where KVM reports > KVM_EXIT_MEMORY_FAULT and fills run->memory_fault on _any_ unresolved > fault, KVM returns "-EFAULT" (-1 with errno == EFAULT from userspace's > perspective), not '0'! Due to historical baggage within KVM, exiting to > userspace with '0' from deep callstacks, e.g. in emulation paths, is > infeasible as doing so would require a near-complete overhaul of KVM, > whereas KVM already propagates -errno return codes to userspace even when > the -errno originated in a low level helper. > > Report the gpa+size instead of a single gfn even though the initial usage > is expected to always report single pages. It's entirely possible, likely > even, that KVM will someday support sub-page granularity faults, e.g. > Intel's sub-page protection feature allows for additional protections at > 128-byte granularity. > > Link: https://lore.kernel.org/all/20230908222905.1321305-5-amoorthy@xxxxxxxxxx > Link: https://lore.kernel.org/all/ZQ3AmLO2SYv3DszH@xxxxxxxxxx > Cc: Anish Moorthy <amoorthy@xxxxxxxxxx> > Cc: David Matlack <dmatlack@xxxxxxxxxx> > Suggested-by: Sean Christopherson <seanjc@xxxxxxxxxx> > Co-developed-by: Yu Zhang <yu.c.zhang@xxxxxxxxxxxxxxx> > Signed-off-by: Yu Zhang <yu.c.zhang@xxxxxxxxxxxxxxx> > Signed-off-by: Chao Peng <chao.p.peng@xxxxxxxxxxxxxxx> > Co-developed-by: Sean Christopherson <seanjc@xxxxxxxxxx> > Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx> > Reviewed-by: Paolo Bonzini <pbonzini@xxxxxxxxxx> > Message-Id: <20231027182217.3615211-10-seanjc@xxxxxxxxxx> > Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx> > --- Reviewed-by: Fuad Tabba <tabba@xxxxxxxxxx> Tested-by: Fuad Tabba <tabba@xxxxxxxxxx> Cheers, /fuad > Documentation/virt/kvm/api.rst | 41 ++++++++++++++++++++++++++++++++++ > arch/x86/kvm/x86.c | 1 + > include/linux/kvm_host.h | 11 +++++++++ > include/uapi/linux/kvm.h | 8 +++++++ > 4 files changed, 61 insertions(+) > > diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst > index bdea1423c5f8..481fb0e2ce90 100644 > --- a/Documentation/virt/kvm/api.rst > +++ b/Documentation/virt/kvm/api.rst > @@ -6846,6 +6846,26 @@ array field represents return values. The userspace should update the return > values of SBI call before resuming the VCPU. For more details on RISC-V SBI > spec refer, https://github.com/riscv/riscv-sbi-doc. > > +:: > + > + /* KVM_EXIT_MEMORY_FAULT */ > + struct { > + __u64 flags; > + __u64 gpa; > + __u64 size; > + } memory_fault; > + > +KVM_EXIT_MEMORY_FAULT indicates the vCPU has encountered a memory fault that > +could not be resolved by KVM. The 'gpa' and 'size' (in bytes) describe the > +guest physical address range [gpa, gpa + size) of the fault. The 'flags' field > +describes properties of the faulting access that are likely pertinent. > +Currently, no flags are defined. > + > +Note! KVM_EXIT_MEMORY_FAULT is unique among all KVM exit reasons in that it > +accompanies a return code of '-1', not '0'! errno will always be set to EFAULT > +or EHWPOISON when KVM exits with KVM_EXIT_MEMORY_FAULT, userspace should assume > +kvm_run.exit_reason is stale/undefined for all other error numbers. > + > :: > > /* KVM_EXIT_NOTIFY */ > @@ -7880,6 +7900,27 @@ This capability is aimed to mitigate the threat that malicious VMs can > cause CPU stuck (due to event windows don't open up) and make the CPU > unavailable to host or other VMs. > > +7.34 KVM_CAP_MEMORY_FAULT_INFO > +------------------------------ > + > +:Architectures: x86 > +:Returns: Informational only, -EINVAL on direct KVM_ENABLE_CAP. > + > +The presence of this capability indicates that KVM_RUN will fill > +kvm_run.memory_fault if KVM cannot resolve a guest page fault VM-Exit, e.g. if > +there is a valid memslot but no backing VMA for the corresponding host virtual > +address. > + > +The information in kvm_run.memory_fault is valid if and only if KVM_RUN returns > +an error with errno=EFAULT or errno=EHWPOISON *and* kvm_run.exit_reason is set > +to KVM_EXIT_MEMORY_FAULT. > + > +Note: Userspaces which attempt to resolve memory faults so that they can retry > +KVM_RUN are encouraged to guard against repeatedly receiving the same > +error/annotated fault. > + > +See KVM_EXIT_MEMORY_FAULT for more information. > + > 8. Other capabilities. > ====================== > > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c > index 7b389f27dffc..8f9d8939b63b 100644 > --- a/arch/x86/kvm/x86.c > +++ b/arch/x86/kvm/x86.c > @@ -4625,6 +4625,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext) > case KVM_CAP_ENABLE_CAP: > case KVM_CAP_VM_DISABLE_NX_HUGE_PAGES: > case KVM_CAP_IRQFD_RESAMPLE: > + case KVM_CAP_MEMORY_FAULT_INFO: > r = 1; > break; > case KVM_CAP_EXIT_HYPERCALL: > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h > index 4e741ff27af3..96aa930536b1 100644 > --- a/include/linux/kvm_host.h > +++ b/include/linux/kvm_host.h > @@ -2327,4 +2327,15 @@ static inline void kvm_account_pgtable_pages(void *virt, int nr) > /* Max number of entries allowed for each kvm dirty ring */ > #define KVM_DIRTY_RING_MAX_ENTRIES 65536 > > +static inline void kvm_prepare_memory_fault_exit(struct kvm_vcpu *vcpu, > + gpa_t gpa, gpa_t size) > +{ > + vcpu->run->exit_reason = KVM_EXIT_MEMORY_FAULT; > + vcpu->run->memory_fault.gpa = gpa; > + vcpu->run->memory_fault.size = size; > + > + /* Flags are not (yet) defined or communicated to userspace. */ > + vcpu->run->memory_fault.flags = 0; > +} > + > #endif > diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h > index 308cc70bd6ab..59010a685007 100644 > --- a/include/uapi/linux/kvm.h > +++ b/include/uapi/linux/kvm.h > @@ -275,6 +275,7 @@ struct kvm_xen_exit { > #define KVM_EXIT_RISCV_CSR 36 > #define KVM_EXIT_NOTIFY 37 > #define KVM_EXIT_LOONGARCH_IOCSR 38 > +#define KVM_EXIT_MEMORY_FAULT 39 > > /* For KVM_EXIT_INTERNAL_ERROR */ > /* Emulate instruction failed. */ > @@ -528,6 +529,12 @@ struct kvm_run { > #define KVM_NOTIFY_CONTEXT_INVALID (1 << 0) > __u32 flags; > } notify; > + /* KVM_EXIT_MEMORY_FAULT */ > + struct { > + __u64 flags; > + __u64 gpa; > + __u64 size; > + } memory_fault; > /* Fix the size of the union. */ > char padding[256]; > }; > @@ -1212,6 +1219,7 @@ struct kvm_ppc_resize_hpt { > #define KVM_CAP_ARM_SUPPORTED_BLOCK_SIZES 229 > #define KVM_CAP_ARM_SUPPORTED_REG_MASK_RANGES 230 > #define KVM_CAP_USER_MEMORY2 231 > +#define KVM_CAP_MEMORY_FAULT_INFO 232 > > #ifdef KVM_CAP_IRQ_ROUTING > > -- > 2.39.1 > >
