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Date:   Thu, 11 Aug 2022 20:03:11 +0000
From:   Sean Christopherson <seanjc@google.com>
To:     "H. Peter Anvin" <hpa@zytor.com>
Cc:     Adam Dunlap <acdunlap@google.com>,
        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,
        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>, linux-kernel@vger.kernel.org,
        llvm@lists.linux.dev, Jacob Xu <jacobhxu@google.com>,
        Alper Gun <alpergun@google.com>, Marc Orr <marcorr@google.com>
Subject: Re: [PATCH] x86/asm: Force native_apic_mem_read to use mov
Message-ID: <YvVgfx+hah/V+r5n@google.com>
References: <20220811180010.3067457-1-acdunlap@google.com>
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 <AB38B5DA-D3AC-4568-A8DD-FC281DDE261A@zytor.com>
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On Thu, Aug 11, 2022, H. Peter Anvin wrote:
> On August 11, 2022 12:27:10 PM PDT, Sean Christopherson <seanjc@google.com> wrote:
> >On Thu, Aug 11, 2022, Adam Dunlap 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.
> >> 
> >> Signed-off-by: Adam Dunlap <acdunlap@google.com>
> >> Reviewed-by: Marc Orr <marcorr@google.com>
> >> Reviewed-by: Jacob Xu <jacobhxu@google.com>
> >> ---
> >>  arch/x86/include/asm/apic.h | 13 ++++++++++++-
> >>  1 file changed, 12 insertions(+), 1 deletion(-)
> >> 
> >> diff --git a/arch/x86/include/asm/apic.h b/arch/x86/include/asm/apic.h
> >> index 3415321c8240..281db79e76a9 100644
> >> --- a/arch/x86/include/asm/apic.h
> >> +++ b/arch/x86/include/asm/apic.h
> >> @@ -109,7 +109,18 @@ static inline void native_apic_mem_write(u32 reg, u32 v)
> >>  
> >>  static inline u32 native_apic_mem_read(u32 reg)
> >>  {
> >> -	return *((volatile u32 *)(APIC_BASE + reg));
> >> +	volatile u32 *addr = (volatile u32 *)(APIC_BASE + reg);
> >> +	u32 out;
> >> +
> >> +	/*
> >> +	 * Functionally, what we want to do is simply return *addr. However,
> >> +	 * this accesses an MMIO which may need to be emulated in some cases.
> >> +	 * The emulator doesn't necessarily support all instructions, so we
> >> +	 * force the read from addr to use a mov instruction.
> >> +	 */
> >> +	asm_inline("movl %1, %0" : "=r"(out) : "m"(*addr));
> >> +
> >> +	return out;
> >
> >Can't this just be:
> >
> >	return readl((void __iomem *)(APIC_BASE + reg));
> 
> The very point of the patch is to force a specific instruction sequence.

Yes, and that specific emulator-friendly instruction also has to be forced for all
of the core x86 read/write MMIO helpers.  And it's also possible for MMIO read/write
to be enlightened to skip the MOV and go straight to #VMGEXIT, i.e. the xAPIC code
shouldn't assume MOV is the best/only option (ignoring the handling of the P54C
erratum in the write path).