Date:   Thu, 29 Jun 2023 13:31:43 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Kai Huang <kai.huang@...el.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     linux-mm@...ck.org, x86@...nel.org, dave.hansen@...el.com,
        kirill.shutemov@...ux.intel.com, tony.luck@...el.com,
        peterz@...radead.org, tglx@...utronix.de, bp@...en8.de,
        mingo@...hat.com, hpa@...or.com, seanjc@...gle.com,
        pbonzini@...hat.com, dan.j.williams@...el.com,
        rafael.j.wysocki@...el.com, ashok.raj@...el.com,
        reinette.chatre@...el.com, len.brown@...el.com, ak@...ux.intel.com,
        isaku.yamahata@...el.com, ying.huang@...el.com, chao.gao@...el.com,
        sathyanarayanan.kuppuswamy@...ux.intel.com, nik.borisov@...e.com,
        bagasdotme@...il.com, sagis@...gle.com, imammedo@...hat.com
Subject: Re: [PATCH v12 07/22] x86/virt/tdx: Add skeleton to enable TDX on
 demand

On 26.06.23 16:12, 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@...el.com>
> ---
> 
> v11 -> v12:
>   - Simplified TDX module global init and lp init status tracking (David).
>   - Added comment around try_init_module_global() for using
>     raw_spin_lock() (Dave).
>   - Added one sentence to changelog to explain why to expose tdx_enable()
>     and tdx_cpu_enable() (Dave).
>   - Simplifed comments around tdx_enable() and tdx_cpu_enable() to use
>     lockdep_assert_*() instead. (Dave)
>   - Removed redundent "TDX" in error message (Dave).
> 
> v10 -> v11:
>   - Return -NODEV instead of -EINVAL when CONFIG_INTEL_TDX_HOST is off.
>   - Return the actual error code for tdx_enable() instead of -EINVAL.
>   - Added Isaku's Reviewed-by.
> 
> v9 -> v10:
>   - Merged the patch to handle per-cpu initialization to this patch to
>     tell the story better.
>   - Changed how to handle the per-cpu initialization to only provide a
>     tdx_cpu_enable() function to let the user of TDX to do it when the
>     user wants to run TDX code on a certain cpu.
>   - Changed tdx_enable() to not call cpus_read_lock() explicitly, but
>     call lockdep_assert_cpus_held() to assume the caller has done that.
>   - Improved comments around tdx_enable() and tdx_cpu_enable().
>   - Improved changelog to tell the story better accordingly.
> 
> v8 -> v9:
>   - Removed detailed TODO list in the changelog (Dave).
>   - Added back steps to do module global initialization and per-cpu
>     initialization in the TODO list comment.
>   - Moved the 'enum tdx_module_status_t' from tdx.c to local tdx.h
> 
> v7 -> v8:
>   - Refined changelog (Dave).
>   - Removed "all BIOS-enabled cpus" related code (Peter/Thomas/Dave).
>   - Add a "TODO list" comment in init_tdx_module() to list all steps of
>     initializing the TDX Module to tell the story (Dave).
>   - Made tdx_enable() unverisally return -EINVAL, and removed nonsense
>     comments (Dave).
>   - Simplified __tdx_enable() to only handle success or failure.
>   - TDX_MODULE_SHUTDOWN -> TDX_MODULE_ERROR
>   - Removed TDX_MODULE_NONE (not loaded) as it is not necessary.
>   - Improved comments (Dave).
>   - Pointed out 'tdx_module_status' is software thing (Dave).
> 
> v6 -> v7:
>   - No change.
> 
> v5 -> v6:
>   - Added code to set status to TDX_MODULE_NONE if TDX module is not
>     loaded (Chao)
>   - Added Chao's Reviewed-by.
>   - Improved comments around cpus_read_lock().
> 
> - v3->v5 (no feedback on v4):
>   - Removed the check that SEAMRR and TDX KeyID have been detected on
>     all present cpus.
>   - Removed tdx_detect().
>   - Added num_online_cpus() to MADT-enabled CPUs check within the CPU
>     hotplug lock and return early with error message.
>   - Improved dmesg printing for TDX module detection and initialization.
> 
> 
> ---
>   arch/x86/include/asm/tdx.h  |   4 +
>   arch/x86/virt/vmx/tdx/tdx.c | 162 ++++++++++++++++++++++++++++++++++++
>   arch/x86/virt/vmx/tdx/tdx.h |  13 +++
>   3 files changed, 179 insertions(+)
> 
> diff --git a/arch/x86/include/asm/tdx.h b/arch/x86/include/asm/tdx.h
> index 4dfe2e794411..d8226a50c58c 100644
> --- a/arch/x86/include/asm/tdx.h
> +++ b/arch/x86/include/asm/tdx.h
> @@ -97,8 +97,12 @@ static inline long tdx_kvm_hypercall(unsigned int nr, unsigned long p1,
>   
>   #ifdef CONFIG_INTEL_TDX_HOST
>   bool platform_tdx_enabled(void);
> +int tdx_cpu_enable(void);
> +int tdx_enable(void);
>   #else	/* !CONFIG_INTEL_TDX_HOST */
>   static inline bool platform_tdx_enabled(void) { return false; }
> +static inline int tdx_cpu_enable(void) { return -ENODEV; }
> +static inline int tdx_enable(void)  { return -ENODEV; }
>   #endif	/* CONFIG_INTEL_TDX_HOST */
>   
>   #endif /* !__ASSEMBLY__ */
> diff --git a/arch/x86/virt/vmx/tdx/tdx.c b/arch/x86/virt/vmx/tdx/tdx.c
> index 141d12376c4d..29ca18f66d61 100644
> --- a/arch/x86/virt/vmx/tdx/tdx.c
> +++ b/arch/x86/virt/vmx/tdx/tdx.c
> @@ -13,6 +13,10 @@
>   #include <linux/errno.h>
>   #include <linux/printk.h>
>   #include <linux/smp.h>
> +#include <linux/cpu.h>
> +#include <linux/spinlock.h>
> +#include <linux/percpu-defs.h>
> +#include <linux/mutex.h>
>   #include <asm/msr-index.h>
>   #include <asm/msr.h>
>   #include <asm/archrandom.h>
> @@ -23,6 +27,13 @@ static u32 tdx_global_keyid __ro_after_init;
>   static u32 tdx_guest_keyid_start __ro_after_init;
>   static u32 tdx_nr_guest_keyids __ro_after_init;
>   
> +static bool tdx_global_initialized;
> +static DEFINE_RAW_SPINLOCK(tdx_global_init_lock);
> +static DEFINE_PER_CPU(bool, tdx_lp_initialized);
> +
> +static enum tdx_module_status_t tdx_module_status;

Why can't you switch to a simple bool here as well?

It's either initialized or uninitialized. If uninitialized and you get 
an error, leave it uninitialized. The next caller will try again and 
fail again.

-- 
Cheers,

David / dhildenb

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