linux-kernel - Re: [PATCH v15 17/23] x86/kexec: Flush cache of TDX private memory

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Message-ID: <cfea7192-4b29-46f9-a500-149121f493c8@intel.com>
Date:   Mon, 27 Nov 2023 10:13:45 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Kai Huang <kai.huang@...el.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     x86@...nel.org, kirill.shutemov@...ux.intel.com,
        peterz@...radead.org, tony.luck@...el.com, tglx@...utronix.de,
        bp@...en8.de, mingo@...hat.com, hpa@...or.com, seanjc@...gle.com,
        pbonzini@...hat.com, rafael@...nel.org, david@...hat.com,
        dan.j.williams@...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 v15 17/23] x86/kexec: Flush cache of TDX private memory

On 11/9/23 03:55, Kai Huang wrote:
...
> --- a/arch/x86/kernel/reboot.c
> +++ b/arch/x86/kernel/reboot.c
> @@ -31,6 +31,7 @@
>  #include <asm/realmode.h>
>  #include <asm/x86_init.h>
>  #include <asm/efi.h>
> +#include <asm/tdx.h>
>  
>  /*
>   * Power off function, if any
> @@ -741,6 +742,20 @@ void native_machine_shutdown(void)
>  	local_irq_disable();
>  	stop_other_cpus();
>  #endif
> +	/*
> +	 * stop_other_cpus() has flushed all dirty cachelines of TDX
> +	 * private memory on remote cpus.  Unlike SME, which does the
> +	 * cache flush on _this_ cpu in the relocate_kernel(), flush
> +	 * the cache for _this_ cpu here.  This is because on the
> +	 * platforms with "partial write machine check" erratum the
> +	 * kernel needs to convert all TDX private pages back to normal
> +	 * before booting to the new kernel in kexec(), and the cache
> +	 * flush must be done before that.  If the kernel took SME's way,
> +	 * it would have to muck with the relocate_kernel() assembly to
> +	 * do memory conversion.
> +	 */
> +	if (platform_tdx_enabled())
> +		native_wbinvd();

Why can't the TDX host code just set host_mem_enc_active=1?

Sure, you'll end up *using* the SME WBINVD support, but then you don't
have two different WBINVD call sites.  You also don't have to mess with
a single line of assembly.