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From:   Alex =?utf-8?Q?Benn=C3=A9e?= <alex.bennee@linaro.org>
To:     Chao Peng <chao.p.peng@linux.intel.com>
Cc:     kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, linux-fsdevel@vger.kernel.org,
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Subject: Re: [PATCH v9 3/8] KVM: Add KVM_EXIT_MEMORY_FAULT exit
Date:   Tue, 15 Nov 2022 16:56:12 +0000
In-reply-to: <20221025151344.3784230-4-chao.p.peng@linux.intel.com>
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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 userspac=
e 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.
>=20=20
> +::
> +
> +		/* 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 user=
space
> +may handle the error and return to KVM to retry the previous memory acce=
ss.
> +
>  ::
>=20=20
>      /* 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
>=20=20
>  /* 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];
>  	};


--=20
Alex Benn=C3=A9e