Copying Zhenzhong Duan as my point relates to the proposed libvirt
TDX patches.
On Thu, Feb 29, 2024 at 01:36:46AM -0500, Xiaoyao Li wrote:
> Bit 28 of TD attribute, named SEPT_VE_DISABLE. When set to 1, it disables
> EPT violation conversion to #VE on guest TD access of PENDING pages.
>
> Some guest OS (e.g., Linux TD guest) may require this bit as 1.
> Otherwise refuse to boot.
>
> Add sept-ve-disable property for tdx-guest object, for user to configure
> this bit.
>
> Signed-off-by: Xiaoyao Li <xiaoyao.li@xxxxxxxxx>
> Acked-by: Gerd Hoffmann <kraxel@xxxxxxxxxx>
> Acked-by: Markus Armbruster <armbru@xxxxxxxxxx>
> ---
> Changes in v4:
> - collect Acked-by from Markus
>
> Changes in v3:
> - update the comment of property @sept-ve-disable to make it more
> descriptive and use new format. (Daniel and Markus)
> ---
> qapi/qom.json | 7 ++++++-
> target/i386/kvm/tdx.c | 24 ++++++++++++++++++++++++
> 2 files changed, 30 insertions(+), 1 deletion(-)
>
> diff --git a/qapi/qom.json b/qapi/qom.json
> index 220cc6c98d4b..89ed89b9b46e 100644
> --- a/qapi/qom.json
> +++ b/qapi/qom.json
> @@ -900,10 +900,15 @@
> #
> # Properties for tdx-guest objects.
> #
> +# @sept-ve-disable: toggle bit 28 of TD attributes to control disabling
> +# of EPT violation conversion to #VE on guest TD access of PENDING
> +# pages. Some guest OS (e.g., Linux TD guest) may require this to
> +# be set, otherwise they refuse to boot.
> +#
> # Since: 9.0
> ##
> { 'struct': 'TdxGuestProperties',
> - 'data': { }}
> + 'data': { '*sept-ve-disable': 'bool' } }
So this exposes a single boolean property that gets mapped into one
specific bit in the TD attributes:
> +
> +static void tdx_guest_set_sept_ve_disable(Object *obj, bool value, Error **errp)
> +{
> + TdxGuest *tdx = TDX_GUEST(obj);
> +
> + if (value) {
> + tdx->attributes |= TDX_TD_ATTRIBUTES_SEPT_VE_DISABLE;
> + } else {
> + tdx->attributes &= ~TDX_TD_ATTRIBUTES_SEPT_VE_DISABLE;
> + }
> +}
If I look at the documentation for TD attributes
https://download.01.org/intel-sgx/latest/dcap-latest/linux/docs/Intel_TDX_DCAP_Quoting_Library_API.pdf
Section "A.3.4. TD Attributes"
I see "TD attributes" is a 64-bit int, with 5 bits currently
defined "DEBUG", "SEPT_VE_DISABLE", "PKS", "PL", "PERFMON",
and the rest currently reserved for future use. This makes me
wonder about our modelling approach into the future ?
For the AMD SEV equivalent we've just directly exposed the whole
field as an int:
'policy' : 'uint32',
For the proposed SEV-SNP patches, the same has been done again
https://lists.nongnu.org/archive/html/qemu-devel/2024-06/msg00536.html
'*policy': 'uint64',
The advantage of exposing individual booleans is that it is
self-documenting at the QAPI level, but the disadvantage is
that every time we want to expose ability to control a new
bit in the policy we have to modify QEMU, libvirt, the mgmt
app above libvirt, and whatever tools the end user has to
talk to the mgmt app.
If we expose a policy int, then newly defined bits only require
a change in QEMU, and everything above QEMU will already be
capable of setting it.
In fact if I look at the proposed libvirt patches, they have
proposed just exposing a policy "int" field in the XML, which
then has to be unpacked to set the individual QAPI booleans
https://lists.libvirt.org/archives/list/devel@xxxxxxxxxxxxxxxxx/message/WXWXEESYUA77DP7YIBP55T2OPSVKV5QW/
On balance, I think it would be better if QEMU just exposed
the raw TD attributes policy as an uint64 at QAPI, instead
of trying to unpack it to discrete bool fields. This gives
consistency with SEV and SEV-SNP, and with what's proposed
at the libvirt level, and minimizes future changes when
more policy bits are defined.
> +
> /* tdx guest */
> OBJECT_DEFINE_TYPE_WITH_INTERFACES(TdxGuest,
> tdx_guest,
> @@ -529,6 +549,10 @@ static void tdx_guest_init(Object *obj)
> qemu_mutex_init(&tdx->lock);
>
> tdx->attributes = 0;
> +
> + object_property_add_bool(obj, "sept-ve-disable",
> + tdx_guest_get_sept_ve_disable,
> + tdx_guest_set_sept_ve_disable);
> }
>
> static void tdx_guest_finalize(Object *obj)
> --
> 2.34.1
>
With regards,
Daniel
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