linux-kernel - Re: [RFC PATCH 6/7] x86/kexec: Debugging support: Dump registers on exception

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Message-ID: <07117fe234cb1396a59bdb815ada26a90e02b394.camel@infradead.org>
Date: Tue, 05 Nov 2024 13:29:54 -0800
From: David Woodhouse <dwmw2@...radead.org>
To: "H. Peter Anvin" <hpa@...or.com>, "peterz@...radead.org"
 <peterz@...radead.org>, "kexec@...ts.infradead.org"
 <kexec@...ts.infradead.org>,  "jpoimboe@...nel.org" <jpoimboe@...nel.org>
Cc: "horms@...nel.org" <horms@...nel.org>, "x86@...nel.org"
 <x86@...nel.org>,  "bp@...en8.de" <bp@...en8.de>, "mingo@...hat.com"
 <mingo@...hat.com>, "tglx@...utronix.de" <tglx@...utronix.de>,
 "kai.huang@...el.com" <kai.huang@...el.com>, 
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
 "nik.borisov@...e.com" <nik.borisov@...e.com>, 
 "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>
Subject: Re:  [RFC PATCH 6/7] x86/kexec: Debugging support: Dump registers
 on exception

On Tue, 2024-11-05 at 12:50 -0800, H. Peter Anvin wrote:
> On November 5, 2024 12:38:10 PM PST, "Woodhouse, David" <dwmw@...zon.co.uk> wrote:
> > On Sun, 2024-11-03 at 05:35 +0000, David Woodhouse wrote:
> > > 
> > > +
> > > +/* Print the byte in %bl, clobber %rax */
> > > +SYM_CODE_START_LOCAL_NOALIGN(pr_byte)
> > > +       movb    %bl, %al
> > > +       nop
> > > +       andb    $0x0f, %al
> > > +       addb    $0x30, %al
> > > +       cmpb    $0x3a, %al
> > > +       jb      1f
> > > +       addb    $('a' - '0' - 10), %al
> > > +1:     pr_char
> > > +       ANNOTATE_UNRET_SAFE
> > > +       ret
> > > +SYM_CODE_END(pr_byte)
> > > +
> > 
> > Obviously that function name (and comment) are wrong; fixed in my tree.
> > at
> > https://git.infradead.org/users/dwmw2/linux.git/shortlog/refs/heads/kexec-debug
> > 
> > This function (and also pr_qword) are also what objtool is complaining
> > about:
> > 
> > vmlinux.o: warning: objtool: relocate_range+0x2f6: unreachable instruction
> > vmlinux.o: warning: objtool: relocate_range+0x305: unreachable instruction
> > 
> > I don't quite see why, because pr_qword() quite blatantly calls
> > pr_nyblle(), as it's now named. And exc_handler() repeatedly calls
> > pr_qword().
> > 
> > But most of the objtool annotations I've added here were just to make
> > it shut up and build, without much though. Peter, Josh, any chance you
> > can help me fix it up please?
> > 
> > It would also be really useful if objtool would let me have data inside
> > a "code" segment, without complaining that it can't decode it as
> > instructions — and without also failing to decode the first instruction
> > of the *subsequent* function. I've put the GDT at the end to work
> > around that, but it's a bit nasty.
> 
> code in the data *section* or *segment*? Either is nasty, though. That's what .rodata is for.

This is the relocate_kernel() function in
arch/x86/kernel/relocate_kernel_64.S

It's copied into a separate page, called (in its original location) as
a simple function from the kernel, changes %cr3 to set of identity-
mapped page tables and jumps to its *identity-mapped* address, then
copies all the right pages for kexec and jumps into the new kernel.

So it's all in a single page, and currently it plays nasty tricks to
store data after the code. Perhaps it *should* have its own code and
data sections and a linker script to keep them together...

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