Re: [PATCH v9 3/8] KVM: Add KVM_EXIT_MEMORY_FAULT exit

[Alex Bennée]

Chao Peng <chao.p.peng@linux.intel.com> writes:

> On Tue, Nov 15, 2022 at 04:56:12PM +0000, Alex Bennée wrote:
>> 
>> 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?
>
> 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@linux.intel.com
> [2] https://lore.kernel.org/all/YKxJLcg%2FWomPE422@google.com
>
>> 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