On Tue, Nov 15, 2022 at 04:56:12PM +0000, Alex Bennée wrote: > > Chao Peng <chao.p.peng@xxxxxxxxxxxxxxx> writes: > > > This new KVM exit allows userspace to handle memory-related errors. It > > indicates an error happens in KVM at guest memory range [gpa, gpa+size). > > The flags includes additional information for userspace to handle the > > error. Currently bit 0 is defined as 'private memory' where '1' > > indicates error happens due to private memory access and '0' indicates > > error happens due to shared memory access. > > > > When private memory is enabled, this new exit will be used for KVM to > > exit to userspace for shared <-> private memory conversion in memory > > encryption usage. In such usage, typically there are two kind of memory > > conversions: > > - explicit conversion: happens when guest explicitly calls into KVM > > to map a range (as private or shared), KVM then exits to userspace > > to perform the map/unmap operations. > > - implicit conversion: happens in KVM page fault handler where KVM > > exits to userspace for an implicit conversion when the page is in a > > different state than requested (private or shared). > > > > 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> > > --- > > Documentation/virt/kvm/api.rst | 23 +++++++++++++++++++++++ > > include/uapi/linux/kvm.h | 9 +++++++++ > > 2 files changed, 32 insertions(+) > > > > diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst > > index f3fa75649a78..975688912b8c 100644 > > --- a/Documentation/virt/kvm/api.rst > > +++ b/Documentation/virt/kvm/api.rst > > @@ -6537,6 +6537,29 @@ 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 { > > + #define KVM_MEMORY_EXIT_FLAG_PRIVATE (1 << 0) > > + __u32 flags; > > + __u32 padding; > > + __u64 gpa; > > + __u64 size; > > + } memory; > > + > > +If exit reason is KVM_EXIT_MEMORY_FAULT then it indicates that the VCPU has > > +encountered a memory error which is not handled by KVM kernel module and > > +userspace may choose to handle it. The 'flags' field indicates the memory > > +properties of the exit. > > + > > + - KVM_MEMORY_EXIT_FLAG_PRIVATE - indicates the memory error is caused by > > + private memory access when the bit is set. Otherwise the memory error is > > + caused by shared memory access when the bit is clear. > > What does a shared memory access failure entail? In the context of confidential computing usages, guest can issue a shared memory access while the memory is actually private from the host point of view. This exit with bit 0 cleared gives userspace a chance to convert the private memory to shared memory on host. > > If you envision any other failure modes it might be worth making it > explicit with additional flags. Sean mentioned some more usages[1][]2] other than the memory conversion for confidential usage. But I would leave those flags being added in the future after those usages being well discussed. [1] https://lkml.kernel.org/r/20200617230052.GB27751@xxxxxxxxxxxxxxx [2] https://lore.kernel.org/all/YKxJLcg%2FWomPE422@xxxxxxxxxx > I also wonder if a bitmask makes sense if > there can only be one reason for a failure? Maybe all that is needed is > a reason enum? Tough we only have one reason right now but we still want to leave room for future extension. Enum can express a single value at once well but bitmask makes it possible to express multiple orthogonal flags. Chao > > > + > > +'gpa' and 'size' indicate the memory range the error occurs at. The userspace > > +may handle the error and return to KVM to retry the previous memory access. > > + > > :: > > > > /* KVM_EXIT_NOTIFY */ > > diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h > > index f1ae45c10c94..fa60b032a405 100644 > > --- a/include/uapi/linux/kvm.h > > +++ b/include/uapi/linux/kvm.h > > @@ -300,6 +300,7 @@ struct kvm_xen_exit { > > #define KVM_EXIT_RISCV_SBI 35 > > #define KVM_EXIT_RISCV_CSR 36 > > #define KVM_EXIT_NOTIFY 37 > > +#define KVM_EXIT_MEMORY_FAULT 38 > > > > /* For KVM_EXIT_INTERNAL_ERROR */ > > /* Emulate instruction failed. */ > > @@ -538,6 +539,14 @@ struct kvm_run { > > #define KVM_NOTIFY_CONTEXT_INVALID (1 << 0) > > __u32 flags; > > } notify; > > + /* KVM_EXIT_MEMORY_FAULT */ > > + struct { > > +#define KVM_MEMORY_EXIT_FLAG_PRIVATE (1 << 0) > > + __u32 flags; > > + __u32 padding; > > + __u64 gpa; > > + __u64 size; > > + } memory; > > /* Fix the size of the union. */ > > char padding[256]; > > }; > > > -- > Alex Bennée
