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From: Kevin Loughlin <kevinloughlin@google.com>
Date: Wed, 10 Jan 2024 09:28:21 -0800
Message-ID: <CAGdbjmK3oTE=3PizXrvux2XT1GLMPp=Fto-2ySfSf_Em4Tjnsw@mail.gmail.com>
Subject: Re: [RFC PATCH] x86/sev: x86/sev: enforce PC-relative addressing in clang
To: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>, 
	Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org, 
	"H. Peter Anvin" <hpa@zytor.com>, Andy Lutomirski <luto@kernel.org>, Peter Zijlstra <peterz@infradead.org>, 
	Nathan Chancellor <nathan@kernel.org>, Nick Desaulniers <ndesaulniers@google.com>, 
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	Rick Edgecombe <rick.p.edgecombe@intel.com>, Kees Cook <keescook@chromium.org>, 
	"Masami Hiramatsu (Google)" <mhiramat@kernel.org>, Ze Gao <zegao2021@gmail.com>, 
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	Brijesh Singh <brijesh.singh@amd.com>, Michael Roth <michael.roth@amd.com>, 
	Ashish Kalra <ashish.kalra@amd.com>, Tom Lendacky <thomas.lendacky@amd.com>, 
	Joerg Roedel <jroedel@suse.de>, linux-kernel@vger.kernel.org, llvm@lists.linux.dev, 
	linux-coco@lists.linux.dev, Adam Dunlap <acdunlap@google.com>, 
	Peter Gonda <pgonda@google.com>, Jacob Xu <jacobhxu@google.com>, 
	Sidharth Telang <sidtelang@google.com>
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On Wed, Jan 10, 2024 at 5:37=E2=80=AFAM Kirill A. Shutemov
<kirill.shutemov@linux.intel.com> wrote:
>
> Similar issues was fixed before with fixup_pointer() tricks. Have you
> tried looking this direction.
>
> Relevant thread starting with:
>
> https://lore.kernel.org/all/20210920192341.maue7db4lcbdn46x@box.shutemov.=
name

Yes, I considered using `fixup_pointer()` as well, and I agree that it
remains a valid option to solve this problem. I just now mentioned the
primary downside to using `fixup_pointer()` in my response to Andi
(https://lore.kernel.org/lkml/CAGdbjmJcQeLWroBkbRC-cFQ6OuMHONsGLzpUrrA5k3uc=
8Rzkmw@mail.gmail.com/).
Namely, we would have to pass around `physaddr` from `__startup64()`
to any functions wanting to use `fixup_pointer()`). However, we should
be able to get around needing `physaddr` by using something like the
`GET_RIP_RELATIVE_PTR()` macro that I proposed in the same reply
(forthcoming in v2 of this RFC PATCH).