Re: [Xen-devel] [PATCH 4/7] x86/asm: Remove opencoded uses of altinstruction_entry

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 12/02/18 17:46, Roger Pau Monné wrote:
> On Mon, Feb 12, 2018 at 11:23:04AM +0000, Andrew Cooper wrote:
>> diff --git a/xen/arch/x86/x86_64/entry.S b/xen/arch/x86/x86_64/entry.S
>> index 58f652d..bd3819a 100644
>> --- a/xen/arch/x86/x86_64/entry.S
>> +++ b/xen/arch/x86/x86_64/entry.S
>> @@ -557,23 +557,9 @@ handle_exception_saved:
>>          testb $X86_EFLAGS_IF>>8,UREGS_eflags+1(%rsp)
>>          jz    exception_with_ints_disabled
>>  
>> -.Lcr4_pv32_orig:
>> -        jmp   .Lcr4_pv32_done
>> -        .skip (.Lcr4_pv32_alt_end - .Lcr4_pv32_alt) - (. - 
>> .Lcr4_pv32_orig), 0xcc
>> -        .pushsection .altinstr_replacement, "ax"
>> -.Lcr4_pv32_alt:
>> -        mov   VCPU_domain(%rbx),%rax
>> -.Lcr4_pv32_alt_end:
>> -        .section .altinstructions, "a"
>> -        altinstruction_entry .Lcr4_pv32_orig, .Lcr4_pv32_alt, \
>> -                             X86_FEATURE_XEN_SMEP, \
>> -                             (.Lcr4_pv32_alt_end - .Lcr4_pv32_alt), \
>> -                             (.Lcr4_pv32_alt_end - .Lcr4_pv32_alt)
>> -        altinstruction_entry .Lcr4_pv32_orig, .Lcr4_pv32_alt, \
>> -                             X86_FEATURE_XEN_SMAP, \
>> -                             (.Lcr4_pv32_alt_end - .Lcr4_pv32_alt), \
>> -                             (.Lcr4_pv32_alt_end - .Lcr4_pv32_alt)
>> -        .popsection
>> +        ALTERNATIVE_2 "jmp .Lcr4_pv32_done; .skip 2, 0x90", \
>> +            __stringify(mov VCPU_domain(%rbx), %rax), X86_FEATURE_XEN_SMEP, 
>> \
>> +            __stringify(mov VCPU_domain(%rbx), %rax), X86_FEATURE_XEN_SMAP
> What's the point of using __stringify here, isn't it clearer to just
> use "mov ..."?

Assembler macro parameters are miserable.  Spaces and commas are both
separators (unless you've got an integer parameter and some brackets). 
Therefore, the "right" way to do this would be:

ALTERNATIVE_2 "jmp .Lcr4_pv32_done; .skip 2, 0x90", \
    "mov VCPU_domain(%rbx), %rax", X86_FEATURE_XEN_SMEP, \
    "mov VCPU_domain(%rbx), %rax", X86_FEATURE_XEN_SMAP

Except that you also need VCPU_domain to be expanded, hence the
__stringify().

~Andrew

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