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From:   Peter Maydell <peter.maydell@linaro.org>
Date:   Tue, 25 Oct 2022 16:26:18 +0100
Message-ID: <CAFEAcA-=Sc9Sc4oLq13HAFW49ZBw8u6DtN7bf_vjVYX_AAaKSg@mail.gmail.com>
Subject: Re: [PATCH v9 3/8] KVM: Add KVM_EXIT_MEMORY_FAULT exit
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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On Tue, 25 Oct 2022 at 16:21, Chao Peng <chao.p.peng@linux.intel.com> wrote:
>
> 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.
> +
> +'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.
> +

What's the difference between this and a plain old MMIO exit ?
Just that we can specify a wider size and some flags ?

-- PMM