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

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Message-ID: <0D6A1E49-F21B-42AA-BBBF-13BFC308BB1E@zytor.com>
Date:   Thu, 11 Aug 2022 21:40:23 -0700
From:   "H. Peter Anvin" <hpa@...or.com>
To:     Sean Christopherson <seanjc@...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>, 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>, linux-kernel@...r.kernel.org,
        llvm@...ts.linux.dev, Jacob Xu <jacobhxu@...gle.com>,
        Alper Gun <alpergun@...gle.com>, Marc Orr <marcorr@...gle.com>
Subject: Re: [PATCH] x86/asm: Force native_apic_mem_read to use mov

On August 11, 2022 1:03:11 PM PDT, Sean Christopherson <seanjc@...gle.com> wrote:
>On Thu, Aug 11, 2022, H. Peter Anvin wrote:
>> On August 11, 2022 12:27:10 PM PDT, Sean Christopherson <seanjc@...gle.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@...gle.com>
>> >> Reviewed-by: Marc Orr <marcorr@...gle.com>
>> >> Reviewed-by: Jacob Xu <jacobhxu@...gle.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).

That's not reasonable... but xAPIC is "special" enough.