linux-kernel - Re: [PATCH][EFI] Run EFI in physical mode

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Message-ID: <m1iq3ep33n.fsf@fess.ebiederm.org>
Date:	Fri, 13 Aug 2010 15:19:56 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Takao Indoh <indou.takao@...fujitsu.com>
Cc:	linux-kernel@...r.kernel.org, kexec@...ts.infradead.org,
	tglx@...utronix.de, mingo@...hat.com, hpa@...or.com,
	vgoyal@...hat.com, nhorman@...driver.com
Subject: Re: [PATCH][EFI] Run EFI in physical mode

Takao Indoh <indou.takao@...fujitsu.com> writes:

> Hi all,
>
> The attached patch enables EFI to run in physical mode.
>
> Basically EFI is in physical mode at first and it's switched to virtual
> mode after calling SetVirtualAddressMap. By applying this patch, you can
> run EFI always in physical mode. And you can also specify "virtefi" as
> kernel boot parameter to run EFI in virtual mode as before. Note that
> this patch supports only x86_64.
>
> This is needed to run kexec/kdump in EFI-booted system. The following is
> an original discussion. In this thread, I explained that kdump does not
> work because EFI system table is modified by SetVirtualAddressMap. And
> the idea to run EFI in physical mode was proposed. This patch implements
> it.
>
> http://marc.info/?l=linux-kernel&m=128018221820234&w=2
>> When the 1st kernel boots, EFI system table(efi_system_table_t) is
>> modified by SetVirtualAddressMap, which is one of EFI runtime service.
>> This runtime changes physical address in EFI system table to virtual
>> address.
>> 
>> When the 2nd kernel boots, it also receives the same EFI system table,
>> and the address included in it is already virtual address(1st kernel
>> rewrote it).  But 2nd kernel does not know that, 2nd kernel thinks it is
>> a physical address. This causes problems.
>
> Basic idea of this patch is to create EFI own pagetable. This pagetable
> maps physical address of EFI runtime to the virtual address which is the
> same value so that we can call it directly. For example, physical 
> address 0x800000 is mapped to virtual address 0x800000. Before calling
> EFI runtime, cr3 register is switched to this pagetable, and restored
> when we come back from EFI.
>
> Any comments would be appreciated.
>
> Signed-off-by: Takao Indoh <indou.takao@...fujitsu.com>

Acked-by: "Eric W. Biederman" <ebiederm@...ssion.com>

There is what appears to be unneeded redundancy (we need two
implementations of physciall calls into efi?), but that is confined to
the weird efi state.

It is a shame you haven't done the little bit extra to get
efi_pagetable_init working on x86_32.

Overall this seems sane and confined to the x86 efi, and it looks
like further improvements could easily be layered on top of this one.

Eric
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