Re: [PATCH v2 RESEND] x86/asm: Force native_apic_mem_read to use mov

From: Marc Orr <marcorr@google.com>
To: Tom Lendacky <thomas.lendacky@amd.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>,
	Adam Dunlap <acdunlap@google.com>,
	Peter Gonda <pgonda@google.com>,
	 Sean Christopherson <seanjc@google.com>,
	Dave Hansen <dave.hansen@intel.com>,
	 Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	 Dave Hansen <dave.hansen@linux.intel.com>,
	"the arch/x86 maintainers" <x86@kernel.org>,
	 Nathan Chancellor <nathan@kernel.org>,
	Nick Desaulniers <ndesaulniers@google.com>,
	Tom Rix <trix@redhat.com>,
	 "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	 Kuppuswamy Sathyanarayanan
	<sathyanarayanan.kuppuswamy@linux.intel.com>,
	 Andi Kleen <ak@linux.intel.com>, Ben Dooks <ben-linux@fluff.org>,
	 LKML <linux-kernel@vger.kernel.org>,
	llvm@lists.linux.dev,  Jacob Xu <jacobhxu@google.com>,
	Alper Gun <alpergun@google.com>
Subject: Re: [PATCH v2 RESEND] x86/asm: Force native_apic_mem_read to use mov
Date: Thu, 17 Nov 2022 13:23:08 -0800	[thread overview]
Message-ID: <CAA03e5Gb0BDn-e7Th8JPQqrX-EmFW3kGeiKfuFvwEzKe6xRadg@mail.gmail.com> (raw)
In-Reply-To: <81950d38-4d5a-6902-bc2e-327d2800eb58@amd.com>

On Wed, Oct 5, 2022 at 6:29 AM Tom Lendacky <thomas.lendacky@amd.com> wrote:
>
> On 10/3/22 18:11, H. Peter Anvin wrote:
> > On October 3, 2022 4:01:01 PM PDT, Adam Dunlap <acdunlap@google.com> wrote:
> >> Thanks for all the responses. Is the consensus that we should use the
> >> readl function here or instead use inline assembly directly as in the patch
> >> I originally sent out:
> >>
> >> asm_inline("movl %1, %0" : "=r"(out) : "m"(*addr));
> >>
> >> ? The readl function has this exact same code, I'm just not sure
> >> which version fits better stylistically.
> >
> > Is mov with an arbitrary addressing mode still acceptable for whatever is causing this problem?
>
> The acceptable forms of MOV are covered by insn_decode_mmio() in
> arch/x86/lib/insn-eval.c.

Is this blocked on an item? There seems to be consensus that this
patch fixes a bug and is taking the right high-level approach (i.e.,
change the guest code to avoid triggering a sequence that isn't
supported under CVM exception-based emulation). Without something like
this, we weren't able to build the kernel w/ CLANG when it is
configured to run under SEV-ES.

We sent out two versions of the patch. One that does the mov directly
[1] and a second that calls readl [2]. Is one of these two patches
acceptable? Or do we need to follow up on something?

[1] https://lore.kernel.org/lkml/0D6A1E49-F21B-42AA-BBBF-13BFC308BB1E@zytor.com/T/
[2] https://lore.kernel.org/all/20220812183501.3555820-1-acdunlap@google.com/

Thanks,
Marc