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

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Date:   Wed, 14 Sep 2022 12:59:19 +0100
From:   Marc Orr <marcorr@...gle.com>
To:     Peter Gonda <pgonda@...gle.com>
Cc:     Adam Dunlap <acdunlap@...gle.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>,
        "H. Peter Anvin" <hpa@...or.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Tom Rix <trix@...hat.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Sean Christopherson <seanjc@...gle.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, Sep 14, 2022 at 12:13 PM Peter Gonda <pgonda@...gle.com> wrote:
>
> On Thu, Sep 8, 2022 at 6:05 PM Adam Dunlap <acdunlap@...gle.com> wrote:
> >
> > Previously, when compiled with clang, native_apic_mem_read gets inlined
> > into __xapic_wait_icr_idle and optimized to a testl instruction. When
> > run in a VM with SEV-ES enabled, it attempts to emulate this
> > instruction, but the emulator does not support it. Instead, use inline
> > assembly to force native_apic_mem_read to use the mov instruction which
> > is supported by the emulator.
>
> This seems to be an issue with the SEV-ES in guest #VC handler's
> "emulator" right?
>
> If that's the case I think this should be fixed in the #VC handler
> instead of fixing the code that is failing to be emulated. What if
> there are other places where a testl is used and our tests have not
> caught them.

That was my initial reaction too. But we spoke w/ Tom offline (before
sending this) and my understanding was that we really should be using
MOV for MMIO. I've cc'd Tom so he can speak to this directly though.