linux-kernel - Re: [PATCH v2 RESEND] x86/asm: Force native_apic_mem

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Message-ID: <CAA03e5Gb0BDn-e7Th8JPQqrX-EmFW3kGeiKfuFvwEzKe6xRadg@mail.gmail.com>
Date:   Thu, 17 Nov 2022 13:23:08 -0800
From:   Marc Orr <marcorr@...gle.com>
To:     Tom Lendacky <thomas.lendacky@....com>
Cc:     "H. Peter Anvin" <hpa@...or.com>,
        Adam Dunlap <acdunlap@...gle.com>,
        Peter Gonda <pgonda@...gle.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Tom Rix <trix@...hat.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Ben Dooks <ben-linux@...ff.org>,
        LKML <linux-kernel@...r.kernel.org>, llvm@...ts.linux.dev,
        Jacob Xu <jacobhxu@...gle.com>, Alper Gun <alpergun@...gle.com>
Subject: Re: [PATCH v2 RESEND] x86/asm: Force native_apic_mem_read to use mov

On Wed, Oct 5, 2022 at 6:29 AM Tom Lendacky <thomas.lendacky@....com> wrote:
>
> On 10/3/22 18:11, H. Peter Anvin wrote:
> > On October 3, 2022 4:01:01 PM PDT, Adam Dunlap <acdunlap@...gle.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