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SubjectRe: [PATCH v9 3/8] KVM: Add KVM_EXIT_MEMORY_FAULT exit
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Chao Peng <chao.p.peng@linux.intel.com> 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@google.com>
> Co-developed-by: Yu Zhang <yu.c.zhang@linux.intel.com>
> Signed-off-by: Yu Zhang <yu.c.zhang@linux.intel.com>
> Signed-off-by: Chao Peng <chao.p.peng@linux.intel.com>
> ---
> 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?

If you envision any other failure modes it might be worth making it
explicit with additional flags. 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?

> +
> +'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

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