Re: [PATCH v2] x86/cpu: Use SERIALIZE in sync_core() when available

[hpa]

On August 4, 2020 9:48:40 PM PDT, Borislav Petkov <bp@xxxxxxx> wrote:
>On Tue, Aug 04, 2020 at 07:10:59PM -0700, Ricardo Neri wrote:
>> The SERIALIZE instruction gives software a way to force the processor
>to
>> complete all modifications to flags, registers and memory from
>previous
>> instructions and drain all buffered writes to memory before the next
>> instruction is fetched and executed. Thus, it serves the purpose of
>> sync_core(). Use it when available.
>> 
>> Commit 7117f16bf460 ("objtool: Fix ORC vs alternatives") enforced
>stack
>> invariance in alternatives. The iret-to-self does not comply with
>such
>> invariance. Thus, it cannot be used inside alternative code. Instead,
>use
>> an alternative that jumps to SERIALIZE when available.
>> 
>> Cc: Andy Lutomirski <luto@xxxxxxxxxx>
>> Cc: Cathy Zhang <cathy.zhang@xxxxxxxxx>
>> Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
>> Cc: Fenghua Yu <fenghua.yu@xxxxxxxxx>
>> Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
>> Cc: Kyung Min Park <kyung.min.park@xxxxxxxxx>
>> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
>> Cc: "Ravi V. Shankar" <ravi.v.shankar@xxxxxxxxx>
>> Cc: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
>> Cc: linux-edac@xxxxxxxxxxxxxxx
>> Cc: linux-kernel@xxxxxxxxxxxxxxx
>> Suggested-by: Andy Lutomirski <luto@xxxxxxxxxx>
>> Signed-off-by: Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx>
>> ---
>> This is a v2 from my initial submission [1]. The first three patches
>of
>> the series have been merged in Linus' tree. Hence, I am submitting
>only
>> this patch for review.
>> 
>> [1]. https://lkml.org/lkml/2020/7/27/8
>> 
>> Changes since v1:
>>  * Support SERIALIZE using alternative runtime patching.
>>    (Peter Zijlstra, H. Peter Anvin)
>>  * Added a note to specify which version of binutils supports
>SERIALIZE.
>>    (Peter Zijlstra)
>>  * Verified that (::: "memory") is used. (H. Peter Anvin)
>> ---
>>  arch/x86/include/asm/special_insns.h |  2 ++
>>  arch/x86/include/asm/sync_core.h     | 10 +++++++++-
>>  2 files changed, 11 insertions(+), 1 deletion(-)
>> 
>> diff --git a/arch/x86/include/asm/special_insns.h
>b/arch/x86/include/asm/special_insns.h
>> index 59a3e13204c3..25cd67801dda 100644
>> --- a/arch/x86/include/asm/special_insns.h
>> +++ b/arch/x86/include/asm/special_insns.h
>> @@ -10,6 +10,8 @@
>>  #include <linux/irqflags.h>
>>  #include <linux/jump_label.h>
>>  
>> +/* Instruction opcode for SERIALIZE; supported in binutils >= 2.35.
>*/
>> +#define __ASM_SERIALIZE ".byte 0xf, 0x1, 0xe8"
>>  /*
>>   * Volatile isn't enough to prevent the compiler from reordering the
>>   * read/write functions for the control registers and messing
>everything up.
>> diff --git a/arch/x86/include/asm/sync_core.h
>b/arch/x86/include/asm/sync_core.h
>> index fdb5b356e59b..201ea3d9a6bd 100644
>> --- a/arch/x86/include/asm/sync_core.h
>> +++ b/arch/x86/include/asm/sync_core.h
>> @@ -5,15 +5,19 @@
>>  #include <linux/preempt.h>
>>  #include <asm/processor.h>
>>  #include <asm/cpufeature.h>
>> +#include <asm/special_insns.h>
>>  
>>  #ifdef CONFIG_X86_32
>>  static inline void iret_to_self(void)
>>  {
>>  	asm volatile (
>> +		ALTERNATIVE("", "jmp 2f", X86_FEATURE_SERIALIZE)
>>  		"pushfl\n\t"
>>  		"pushl %%cs\n\t"
>>  		"pushl $1f\n\t"
>>  		"iret\n\t"
>> +		"2:\n\t"
>> +		__ASM_SERIALIZE "\n"
>>  		"1:"
>>  		: ASM_CALL_CONSTRAINT : : "memory");
>>  }
>> @@ -23,6 +27,7 @@ static inline void iret_to_self(void)
>>  	unsigned int tmp;
>>  
>>  	asm volatile (
>> +		ALTERNATIVE("", "jmp 2f", X86_FEATURE_SERIALIZE)
>
>Why is this and above stuck inside the asm statement?
>
>Why can't you simply do:
>
>	if (static_cpu_has(X86_FEATURE_SERIALIZE)) {
>		asm volatile(__ASM_SERIALIZE ::: "memory");
>		return;
>	}
>
>on function entry instead of making it more unreadable for no
>particular
>reason?

Because why use an alternative to jump over one instruction?

I personally would prefer to have the IRET put out of line and have the call/jmp replaced by SERIALIZE inline.

-- 
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