On 15/05/2024 12:59 pm, Rick Edgecombe wrote:
From: Isaku Yamahata <isaku.yamahata@xxxxxxxxx>
Introduce a "gfn_shared_mask" field in the kvm_arch structure to record GPA
shared bit and provide address conversion helpers for TDX shared bit of
GPA.
TDX designates a specific GPA bit as the shared bit, which can be either
bit 51 or bit 47 based on configuration.
This GPA shared bit indicates whether the corresponding physical page is
shared (if shared bit set) or private (if shared bit cleared).
- GPAs with shared bit set will be mapped by VMM into conventional EPT,
which is pointed by shared EPTP in TDVMCS, resides in host VMM memory
and is managed by VMM.
- GPAs with shared bit cleared will be mapped by VMM firstly into a
mirrored EPT, which resides in host VMM memory. Changes of the mirrored
EPT are then propagated into a private EPT, which resides outside of host
VMM memory and is managed by TDX module.
Add the "gfn_shared_mask" field to the kvm_arch structure for each VM with
a default value of 0. It will be set to the position of the GPA shared bit
in GFN through TD specific initialization code.
Provide helpers to utilize the gfn_shared_mask to determine whether a GPA
is shared or private, retrieve the GPA shared bit value, and insert/strip
shared bit to/from a GPA.
I am seriously thinking whether we should just abandon this whole
kvm_gfn_shared_mask() thing.
We already have enough mechanisms around private memory and the mapping
of it:
1) Xarray to query whether a given GFN is private or shared;
2) fault->is_private to indicate whether a faulting address is private
or shared;
3) sp->is_private to indicate whether a "page table" is only for private
mapping;
Consider this as 4) -- I also like to have a kvm->arch.has_mirrored_pt
(or a better name) as I replied here:
https://lore.kernel.org/kvm/20240515005952.3410568-17-rick.p.edgecombe@xxxxxxxxx/T/#m49b37658f03e786c6aa43719cbf748215170980d
So I believe we really already have enough mechanisms in the *COMMON*
code for private page/mapping support. I intend to believe the whole
GPA shared bit thing can be hidden in TDX specific operations. If
there's really a need to apply/strip GPA shared bit in the common code,
we can do via kvm_x86_ops callback (I'll review other patches to see).
And btw, I think ...
[...]
+
+/*
+ * default or SEV-SNP TDX: where S = (47 or 51) - 12
+ * gfn_shared_mask 0 S bit
+ * is_private_gpa() always false true if GPA has S bit clear
... this @is_private_gpa(), and ...
+ * gfn_to_shared() nop set S bit
+ * gfn_to_private() nop clear S bit
+ *
+ * fault.is_private means that host page should be gotten from guest_memfd
+ * is_private_gpa() means that KVM MMU should invoke private MMU hooks.
+ */
... this invoking MMU hooks based on @is_private_gpa() makes no sense,
because clearly for SEV-SNP @is_priavate_gpa() isn't report the fact
when the GPA is indeed private, and the MMU hooks should be invoked
based on whether the faulting GPA is private or not, but not this
@is_private_gpa().
