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

[Chao Peng <chao.p.peng@linux.intel.com>]

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.

> 
> 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.

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