Re: [PATCH 2/2] x86/tdx: Convert shared memory back to private on kexec

From: "Huang, Kai" <kai.huang@intel.com>
To: "kirill.shutemov@linux.intel.com"
	<kirill.shutemov@linux.intel.com>,
	"Hansen, Dave" <dave.hansen@intel.com>,
	"bp@alien8.de" <bp@alien8.de>
Cc: "sathyanarayanan.kuppuswamy@linux.intel.com"
	<sathyanarayanan.kuppuswamy@linux.intel.com>,
	"tglx@linutronix.de" <tglx@linutronix.de>,
	"linux-coco@lists.linux.dev" <linux-coco@lists.linux.dev>,
	"kexec@lists.infradead.org" <kexec@lists.infradead.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"Yamahata, Isaku" <isaku.yamahata@intel.com>,
	"x86@kernel.org" <x86@kernel.org>
Subject: Re: [PATCH 2/2] x86/tdx: Convert shared memory back to private on kexec
Date: Wed, 15 Feb 2023 01:53:19 +0000	[thread overview]
Message-ID: <39b41f5b5852c5e9d23acb84a4ed4d9bf8a7c092.camel@intel.com> (raw)
In-Reply-To: <20230213234836.3683-3-kirill.shutemov@linux.intel.com>

> +void tdx_kexec_prepare(bool crash)
> +{
> +	/*
> +	 * Crash kernel may want to see data in the shared buffers.
> +	 * Do not revert them to private on kexec of crash kernel.
> +	 */
> +	if (crash)
> +		return;
> +
> +	/*
> +	 * Walk direct mapping and convert all shared memory back to private,
> +	 * so the target kernel will be able use it normally.
> +	 */
> +	mmap_write_lock(&init_mm);
> +	walk_page_range_novma(&init_mm,
> +			      PAGE_OFFSET,
> +			      PAGE_OFFSET + (max_pfn_mapped << PAGE_SHIFT),
> +			      &unshare_ops, init_mm.pgd, NULL);
> +	mmap_write_unlock(&init_mm);
> +}

Looks the page table walk is done unconditionally when !crash.

I think it's better to check whether this is TDX guest (either this function, or
below in machine_kexec()) and just return early if it's not a TDX guest?

[..]

>  /*
> @@ -312,6 +313,7 @@ void machine_kexec(struct kimage *image)
>  	local_irq_disable();
>  	hw_breakpoint_disable();
>  	cet_disable();
> +	tdx_kexec_prepare(image->type == KEXEC_TYPE_CRASH);
>  
>  	if (image->preserve_context) {
>  #ifdef CONFIG_X86_IO_APIC

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