Re: [PATCH v3 19/19] hostmem: Automatically select set guest-memfd=on for in-place conversion

[Markus Armbruster <armbru@redhat.com>]

Michael Roth <michael.roth@amd.com> writes:

> When in-place conversion is enabled, both the shared memory and private
> memory must come from the same guest_memfd instance. Thus, the
> memory-backend-memfd options must in turn correspond to a guest_memfd
> instance, e.g. guest-memfd=on must be specified. Since there is no
> use-case for enabling in-place conversion without setting
> guest-memfd=on, just set it automatically if in-place conversion is
> enabled (unless guest-memfd has been explicitly set to 'off', in which
> case generate an informative error message).
>
> Also update the 'guest-memfd' documentation to note this behavior.
>
> Signed-off-by: Michael Roth <michael.roth@amd.com>

[...]

> diff --git a/docs/system/guest-memfd.rst b/docs/system/guest-memfd.rst
> index d62a214a91..4477756424 100644
> --- a/docs/system/guest-memfd.rst
> +++ b/docs/system/guest-memfd.rst
> @@ -53,6 +53,27 @@ exposed as an experimental for the time being:
>   * guest_memfd does not currently support the hugetlb=on option
>   * guest_memfd does not currently support Transparent Huge Pages
>  
> +In-place conversion
> +-------------------
> +
> +Newer kernels now support using a guest-memfd instance for both
> +private and shared memory at the same time by re-using the same
> +physical backing pages if a guest converts a GPA range between
> +shared and private. This is known as "in-place conversion".
> +
> +For some confidential computing architectures, like SEV-SNP and
> +TDX, this mode offers better performance due to not needing to
> +deallocate/reallocate between 2 separate pools of shared vs. private
> +memory every time the guest converts memory between shared/private.
> +For other architectures, this is the only supported mode of
> +operation.
> +
> +When in-place conversion is enabled, it is necessary to use the
> +memory-backend-memfd backend with guest-memfd=on as shown in the
> +"Usage" section. Because of this, guest-memfd=auto (which is the

Recommend to make this a link.

> +default) will result in the parameter being automatically enabled
> +if the confidential guest type enables in-place conversion.
> +
>  References
>  ----------
>  
> diff --git a/qapi/qom.json b/qapi/qom.json
> index 84166c2457..5d66e67087 100644
> --- a/qapi/qom.json
> +++ b/qapi/qom.json
> @@ -756,8 +756,9 @@
>  # @guest-memfd: if 'on', use guest-memfd to back the memory region.
>  #     See the :doc:`/system/guest-memfd` documentation for more
>  #     details.  If 'auto', the option will default to 'off'
> -#     currently, but in the future it may result in the option being
> -#     set to 'on' for QEMU configurations that explicitly require it.
> +#     unless the QEMU configuration explicitly requires it to be set,
> +#     e.g. if in-place conversion is enabled for confidential guest
> +#     types like sev-snp-guest.
>  #     (default: auto, since: 11.2)
>  #
>  # Features:

QAPI schema
Acked-by: Markus Armbruster <armbru@redhat.com>