linux-kernel - Re: [PATCH 2/2] Fix efi

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Message-ID: <20160512114346.GE2728@codeblueprint.co.uk>
Date:	Thu, 12 May 2016 12:43:46 +0100
From:	Matt Fleming <matt@...eblueprint.co.uk>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Alex Thorlton <athorlton@....com>, linux-kernel@...r.kernel.org,
	Dimitri Sivanich <sivanich@....com>,
	Russ Anderson <rja@....com>, Mike Travis <travis@....com>,
	Borislav Petkov <bp@...e.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	linux-efi@...r.kernel.org
Subject: Re: [PATCH 2/2] Fix efi_call

On Thu, 12 May, at 08:48:35AM, Ingo Molnar wrote:
> 
> * Alex Thorlton <athorlton@....com> wrote:
> 
> > The efi_call assembly code has a slight error that prevents us from
> > using arguments 7 and higher, which will be passed in on the stack.
> > 
> >         mov (%rsp), %rax
> >         mov 8(%rax), %rax
> > 	...
> >         mov %rax, 40(%rsp)
> > 
> > This code goes and grabs the return address for the current stack frame,
> > and puts it on the stack, next the 5th argument for the EFI runtime
> > call.  Considering the fact that having the return address in that
> > position on the stack makes no sense, I'm guessing that the intent of
> > this code was actually to grab an argument off the stack frame for this
> > call and place it into the frame for the next one.
> > 
> > The small change to that offset (i.e. 8(%rax) to 16(%rax)) ensures that
> > we grab the 7th argument off the stack, and pass it as the 6th argument
> > to the EFI runtime function that we're about to call.  This change gets
> > our EFI runtime calls that need to pass more than 6 arguments working
> > again.
> 
> I suppose the SGI/UV code is the only one using 7 arguments or more? Might make 
> sense to point that out in the changelog.
 
Yeah, I included that info when I applied this patch.