On Wed, Nov 16, 2022 at 07:03:49PM +0000, Alex Bennée wrote: > > Chao Peng <chao.p.peng@xxxxxxxxxxxxxxx> writes: > > > 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. > > I think this should be explicit rather than implied by the absence of > another flag. Sean suggested you might want flags for RWX failures so > maybe something like: > > KVM_MEMORY_EXIT_SHARED_FLAG_READ (1 << 0) > KVM_MEMORY_EXIT_SHARED_FLAG_WRITE (1 << 1) > KVM_MEMORY_EXIT_SHARED_FLAG_EXECUTE (1 << 2) > KVM_MEMORY_EXIT_FLAG_PRIVATE (1 << 3) Yes, but I would not add 'SHARED' to RWX, they are not share memory specific, private memory can also set them once introduced. Thanks, Chao > > which would allow you to signal the various failure modes of the shared > region, or that you had accessed private memory. > > > > >> > >> 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. > > I agree if multiple orthogonal failures can occur at once a bitmask is > the right choice. > > > > > 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 > > > -- > Alex Bennée
