linux-kernel - Re: [PATCH 2/5] x86/pvh: Make PVH entrypoint PIC for x86-64

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Message-ID: <74465ac2-0060-4047-97d7-acadcdff4cd4@amd.com>
Date: Thu, 11 Apr 2024 11:26:11 -0400
From: Jason Andryuk <jason.andryuk@....com>
To: Brian Gerst <brgerst@...il.com>
CC: Juergen Gross <jgross@...e.com>, Boris Ostrovsky
	<boris.ostrovsky@...cle.com>, Thomas Gleixner <tglx@...utronix.de>, Ingo
 Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, Dave Hansen
	<dave.hansen@...ux.intel.com>, <x86@...nel.org>, "H. Peter Anvin"
	<hpa@...or.com>, Stefano Stabellini <sstabellini@...nel.org>, Oleksandr
 Tyshchenko <oleksandr_tyshchenko@...m.com>, Paolo Bonzini
	<pbonzini@...hat.com>, <xen-devel@...ts.xenproject.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/5] x86/pvh: Make PVH entrypoint PIC for x86-64

On 2024-04-10 17:00, Brian Gerst wrote:
> On Wed, Apr 10, 2024 at 3:50 PM Jason Andryuk <jason.andryuk@....com> wrote:

>>          /* 64-bit entry point. */
>>          .code64
>>   1:
>> +       UNWIND_HINT_END_OF_STACK
>> +
>>          /* Set base address in stack canary descriptor. */
>>          mov $MSR_GS_BASE,%ecx
>> -       mov $_pa(canary), %eax
>> +       leal rva(canary)(%ebp), %eax
> 
> Since this is in 64-bit mode, RIP-relative addressing can be used.
> 
>>          xor %edx, %edx
>>          wrmsr
>>
>>          call xen_prepare_pvh
>>
>>          /* startup_64 expects boot_params in %rsi. */
>> -       mov $_pa(pvh_bootparams), %rsi
>> -       mov $_pa(startup_64), %rax
>> +       lea rva(pvh_bootparams)(%ebp), %rsi
>> +       lea rva(startup_64)(%ebp), %rax
> 
> RIP-relative here too.

Yes, thanks for catching that.  With the RIP-relative conversion, there 
is now:
vmlinux.o: warning: objtool: pvh_start_xen+0x10d: relocation to !ENDBR: 
startup_64+0x0

I guess RIP-relative made it visible.  That can be quieted by adding 
ANNOTATE_NOENDBR to startup_64.

Thanks,
Jason