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

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

On Wed, Nov 16, 2022 at 07:03:49PM +0000, Alex Bennée wrote:
> 
> 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)

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@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