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

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Message-ID: <AB38B5DA-D3AC-4568-A8DD-FC281DDE261A@zytor.com>
Date:   Thu, 11 Aug 2022 12:57:34 -0700
From:   "H. Peter Anvin" <hpa@...or.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Adam Dunlap <acdunlap@...gle.com>
CC:     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 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.