Re: [RFC PATCH 00/13][V3] kexec: A new system call to allow in kernel loading

From: "H. Peter Anvin" <hpa@zytor.com>
To: Matt Fleming <matt.fleming@intel.com>
Cc: "mjg59@srcf.ucam.org" <mjg59@srcf.ucam.org>,
	bhe@redhat.com, greg@kroah.com, kexec@lists.infradead.org,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"bp@alien8.de" <bp@alien8.de>,
	ebiederm@xmission.com, "jkosina@suse.cz" <jkosina@suse.cz>,
	chaowang@redhat.com, Dave Young <dyoung@redhat.com>,
	"akpm@linux-foundation.org" <akpm@linux-foundation.org>,
	Vivek Goyal <vgoyal@redhat.com>
Subject: Re: [RFC PATCH 00/13][V3] kexec: A new system call to allow in kernel loading
Date: Fri, 06 Jun 2014 14:00:49 -0700	[thread overview]
Message-ID: <53922C01.4040800@zytor.com> (raw)
In-Reply-To: <CAL01qpsf_-Ae_P-mfmqfu6DYavcb866YUnE4XM_7vdhfcAvMGg@mail.gmail.com>

On 06/06/2014 01:58 PM, Matt Fleming wrote:
> On 6 June 2014 21:37, H. Peter Anvin <hpa@zytor.com> wrote:
>>
>> OK... this is seriously problematic.
>>
>> #if defined(CONFIG_RELOCATABLE) && defined(CONFIG_X86_64) && \
>>         !defined(CONFIG_EFI_MIXED)
>>    /* kernel/boot_param/ramdisk could be loaded above 4g */
>> # define XLF1 XLF_CAN_BE_LOADED_ABOVE_4G
>> #else
>> # define XLF1 0
>> #endif
>>
>> The fact that even compiling with CONFIG_EFI_MIXED disables
>> XLF_CAN_BE_LOADED_ABOVE_4G is really not going to fly.  We should expect
>> CONFIG_EFI_MIXED to be the norm, but *also* should expect that there is
>> a legitimate need to load above 4G.
>>
>> Matt, could you explain why this is necessary?  We need to figure out a
>> way around this.
>>
>> My thinking is that disabling this flag is unnecessary, since a 32-bit
>> EFI loader should not load above the 4G mark anyway, but if I'm confused
>> and there is a more fundamental requirement, then we need to consider
>> that more carefully.
> 
> No, your comments are absolutely correct. I was the one who was
> confused. I found this in the git history,
> 
> commit 7d453eee36ae
> Author: Matt Fleming <matt.fleming@intel.com>
> Date:   Fri Jan 10 18:52:06 2014 +0000
> 
>     x86/efi: Wire up CONFIG_EFI_MIXED
> 
>     Add the Kconfig option and bump the kernel header version so that boot
>     loaders can check whether the handover code is available if they want.
> 
>     The xloadflags field in the bzImage header is also updated to reflect
>     that the kernel supports both entry points by setting both of
>     XLF_EFI_HANDOVER_32 and XLF_EFI_HANDOVER_64 when CONFIG_EFI_MIXED=y.
>     XLF_CAN_BE_LOADED_ABOVE_4G is disabled so that the kernel text is
>     guaranteed to be addressable with 32-bits.
> 
> As you've pointed out above, a 32-bit loader is never going to load
> the kernel above 4G, so we don't need to disable it.
> 
> What's the best way to fix this up? Just undo the change from the above commit?
> 

Yes, presumably (as a separate patch since the actual commit is quite
large.)  The patch needs to have a good description why the original
patch was wrong.

	-hpa

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