Re: [RFC PATCH v2] x86/sev: enforce RIP-relative accesses in early SEV/SME code

From: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
To: Kevin Loughlin <kevinloughlin@google.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>,
	Bill Wendling <morbo@google.com>,
	Justin Stitt <justinstitt@google.com>,
	Tom Lendacky <thomas.lendacky@amd.com>,
	Michael Kelley <mikelley@microsoft.com>,
	Pankaj Gupta <pankaj.gupta@amd.com>,
	Stephen Rothwell <sfr@canb.auug.org.au>,
	Arnd Bergmann <arnd@arndb.de>,
	Steve Rutherford <srutherford@google.com>,
	Alexander Shishkin <alexander.shishkin@linux.intel.com>,
	Hou Wenlong <houwenlong.hwl@antgroup.com>,
	Vegard Nossum <vegard.nossum@oracle.com>,
	Josh Poimboeuf <jpoimboe@kernel.org>,
	Yuntao Wang <ytcoode@gmail.com>,
	Wang Jinchao <wangjinchao@xfusion.com>,
	David Woodhouse <dwmw@amazon.co.uk>,
	Brian Gerst <brgerst@gmail.com>, Hugh Dickins <hughd@google.com>,
	Ard Biesheuvel <ardb@kernel.org>, Joerg Roedel <jroedel@suse.de>,
	Randy Dunlap <rdunlap@infradead.org>,
	Bjorn Helgaas <bhelgaas@google.com>,
	Dionna Glaze <dionnaglaze@google.com>,
	Brijesh Singh <brijesh.singh@amd.com>,
	Michael Roth <michael.roth@amd.com>,
	linux-kernel@vger.kernel.org, llvm@lists.linux.dev,
	linux-coco@lists.linux.dev, Ashish Kalra <ashish.kalra@amd.com>,
	Andi Kleen <ak@linux.intel.com>,
	Adam Dunlap <acdunlap@google.com>,
	Peter Gonda <pgonda@google.com>, Jacob Xu <jacobhxu@google.com>,
	Sidharth Telang <sidtelang@google.com>
Subject: Re: [RFC PATCH v2] x86/sev: enforce RIP-relative accesses in early SEV/SME code
Date: Mon, 15 Jan 2024 13:12:39 +0300	[thread overview]
Message-ID: <20240115101239.nab725vuazvutgw6@box.shutemov.name> (raw)
In-Reply-To: <CAGdbjmLeyPhYfjVHPRa8LgNYjt9-rOPiyCodHoQOkEh9iQhjBg@mail.gmail.com>

On Fri, Jan 12, 2024 at 10:29:36AM -0800, Kevin Loughlin wrote:
> On Fri, Jan 12, 2024 at 4:17 AM Kirill A. Shutemov
> <kirill.shutemov@linux.intel.com> wrote:
> >
> > Can we replace existing fixup_pointer() (and other fixup_*()) with the new
> > thing? I don't think we need two confusing things for the same function.
> 
> Per my tests, yes we can; I replaced the fixup_*() functions with
> GET_RIP_RELATIVE_PTR()/PTR_TO_RIP_RELATIVE_PTR(), and guests with and
> without SEV, SEV-ES, and SEV-SNP all successfully booted under both
> clang and gcc builds.

Okay good.

BTW, do we need both macros? Caller can do &var, right?

If we are okay with single macro, maybe rename it to RIP_RELATIVE_PTR().

One other thing: I see you sprinkle casts to for every use of the macros.
But why? void* can cast to any other pointer without explicit casting.

> I have a slight preference for sending that as a
> separate follow-up commit, but please let me know if you feel
> differently. Thanks.

I'm okay with a separate patch in the same patchset.

> 
> > Also, is there any reason why GET_RIP_RELATIVE_PTR() and
> > PTR_TO_RIP_RELATIVE_PTR() have to be macros? Inline functions would be
> > cleaner.
> 
> I used macros because we need to use both the global variable itself
> and the global variable's string name (obtained via #var in the macro)
> in the inline assembly. As a secondary reason, the macro also avoids
> the need to provide separate functions for each type of variable for
> which we'd like to get RIP-relative pointers (ex: u64, unsigned int,
> unsigned long, etc.).

If we do it only on pointers, wouldn't void * -> void * be enough?

-- 
  Kiryl Shutsemau / Kirill A. Shutemov