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) 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
