Re: [PATCH v14 07/23] x86/virt/tdx: Add skeleton to enable TDX on demand

[Nikolay Borisov]

On 17.10.23 г. 13:14 ч., Kai Huang wrote:
> To enable TDX the kernel needs to initialize TDX from two perspectives:
> 1) Do a set of SEAMCALLs to initialize the TDX module to make it ready
> to create and run TDX guests; 2) Do the per-cpu initialization SEAMCALL
> on one logical cpu before the kernel wants to make any other SEAMCALLs
> on that cpu (including those involved during module initialization and
> running TDX guests).
>
> The TDX module can be initialized only once in its lifetime.  Instead
> of always initializing it at boot time, this implementation chooses an
> "on demand" approach to initialize TDX until there is a real need (e.g
> when requested by KVM).  This approach has below pros:
>
> 1) It avoids consuming the memory that must be allocated by kernel and
> given to the TDX module as metadata (~1/256th of the TDX-usable memory),
> and also saves the CPU cycles of initializing the TDX module (and the
> metadata) when TDX is not used at all.
>
> 2) The TDX module design allows it to be updated while the system is
> running.  The update procedure shares quite a few steps with this "on
> demand" initialization mechanism.  The hope is that much of "on demand"
> mechanism can be shared with a future "update" mechanism.  A boot-time
> TDX module implementation would not be able to share much code with the
> update mechanism.
>
> 3) Making SEAMCALL requires VMX to be enabled.  Currently, only the KVM
> code mucks with VMX enabling.  If the TDX module were to be initialized
> separately from KVM (like at boot), the boot code would need to be
> taught how to muck with VMX enabling and KVM would need to be taught how
> to cope with that.  Making KVM itself responsible for TDX initialization
> lets the rest of the kernel stay blissfully unaware of VMX.
>
> Similar to module initialization, also make the per-cpu initialization
> "on demand" as it also depends on VMX being enabled.
>
> Add two functions, tdx_enable() and tdx_cpu_enable(), to enable the TDX
> module and enable TDX on local cpu respectively.  For now tdx_enable()
> is a placeholder.  The TODO list will be pared down as functionality is
> added.
>
> Export both tdx_cpu_enable() and tdx_enable() for KVM use.
>
> In tdx_enable() use a state machine protected by mutex to make sure the
> initialization will only be done once, as tdx_enable() can be called
> multiple times (i.e. KVM module can be reloaded) and may be called
> concurrently by other kernel components in the future.
>
> The per-cpu initialization on each cpu can only be done once during the
> module's life time.  Use a per-cpu variable to track its status to make
> sure it is only done once in tdx_cpu_enable().
>
> Also, a SEAMCALL to do TDX module global initialization must be done
> once on any logical cpu before any per-cpu initialization SEAMCALL.  Do
> it inside tdx_cpu_enable() too (if hasn't been done).
>
> tdx_enable() can potentially invoke SEAMCALLs on any online cpus.  The
> per-cpu initialization must be done before those SEAMCALLs are invoked
> on some cpu.  To keep things simple, in tdx_cpu_enable(), always do the
> per-cpu initialization regardless of whether the TDX module has been
> initialized or not.  And in tdx_enable(), don't call tdx_cpu_enable()
> but assume the caller has disabled CPU hotplug, done VMXON and
> tdx_cpu_enable() on all online cpus before calling tdx_enable().
>
> Signed-off-by: Kai Huang <kai.huang@xxxxxxxxx>
> Reviewed-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>


With the latest explanation from Kai :

Reviewed-by: Nikolay Borisov <nik.borisov@xxxxxxxx>