linux-kernel - Re: [PATCH v12 19/22] x86/kexec(): Reset TDX private memory on platforms with TDX erratum

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Date:   Wed, 28 Jun 2023 12:20:24 +0300
From:   Nikolay Borisov <nik.borisov@...e.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, david@...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, bagasdotme@...il.com,
        sagis@...gle.com, imammedo@...hat.com
Subject: Re: [PATCH v12 19/22] x86/kexec(): Reset TDX private memory on
 platforms with TDX erratum



On 26.06.23 г. 17:12 ч., Kai Huang wrote:

<snip>



> diff --git a/arch/x86/virt/vmx/tdx/tdx.c b/arch/x86/virt/vmx/tdx/tdx.c
> index 85b24b2e9417..1107f4227568 100644
> --- a/arch/x86/virt/vmx/tdx/tdx.c
> +++ b/arch/x86/virt/vmx/tdx/tdx.c
> @@ -51,6 +51,8 @@ static LIST_HEAD(tdx_memlist);
>   
>   static struct tdmr_info_list tdx_tdmr_list;
>   
> +static atomic_t tdx_may_has_private_mem;
> +
>   /*
>    * Wrapper of __seamcall() to convert SEAMCALL leaf function error code
>    * to kernel error code.  @seamcall_ret and @out contain the SEAMCALL
> @@ -1113,6 +1115,17 @@ static int init_tdx_module(void)
>   	 */
>   	wbinvd_on_all_cpus();
>   
> +	/*
> +	 * Starting from this point the system may have TDX private
> +	 * memory.  Make it globally visible so tdx_reset_memory() only
> +	 * reads TDMRs/PAMTs when they are stable.
> +	 *
> +	 * Note using atomic_inc_return() to provide the explicit memory
> +	 * ordering isn't mandatory here as the WBINVD above already
> +	 * does that.  Compiler barrier isn't needed here either.
> +	 */

If it's not needed, then why use it? Simply do atomic_inc() and instead 
rephrase the comment to state what are the ordering guarantees and how 
they are achieved (i.e by using wbinvd above).

> +	atomic_inc_return(&tdx_may_has_private_mem);
> +
>   	/* Config the key of global KeyID on all packages */
>   	ret = config_global_keyid();
>   	if (ret)
> @@ -1154,6 +1167,15 @@ static int init_tdx_module(void)
>   	 * as suggested by the TDX spec.
>   	 */
>   	tdmrs_reset_pamt_all(&tdx_tdmr_list);
> +	/*
> +	 * No more TDX private pages now, and PAMTs/TDMRs are
> +	 * going to be freed.  Make this globally visible so
> +	 * tdx_reset_memory() can read stable TDMRs/PAMTs.
> +	 *
> +	 * Note atomic_dec_return(), which is an atomic RMW with
> +	 * return value, always enforces the memory barrier.
> +	 */
> +	atomic_dec_return(&tdx_may_has_private_mem);

Make a comment here which either refers to the comment at the increment 
site.

>   out_free_pamts:
>   	tdmrs_free_pamt_all(&tdx_tdmr_list);
>   out_free_tdmrs:
> @@ -1229,6 +1251,63 @@ int tdx_enable(void)
>   }
>   EXPORT_SYMBOL_GPL(tdx_enable);
>   
> +/*
> + * Convert TDX private pages back to normal on platforms with
> + * "partial write machine check" erratum.
> + *
> + * Called from machine_kexec() before booting to the new kernel.
> + */
> +void tdx_reset_memory(void)
> +{
> +	if (!platform_tdx_enabled())
> +		return;
> +
> +	/*
> +	 * Kernel read/write to TDX private memory doesn't
> +	 * cause machine check on hardware w/o this erratum.
> +	 */
> +	if (!boot_cpu_has_bug(X86_BUG_TDX_PW_MCE))
> +		return;
> +
> +	/* Called from kexec() when only rebooting cpu is alive */
> +	WARN_ON_ONCE(num_online_cpus() != 1);
> +
> +	if (!atomic_read(&tdx_may_has_private_mem))
> +		return;

I think a comment is warranted here explicitly calling our the ordering 
requirement/guarantees. Actually this is a non-rmw operation so it 
doesn't have any bearing on the ordering/implicit mb's achieved at the 
"increment" site.

<snip>