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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 12.02.18 at 13:30, <wei.liu2@xxxxxxxxxx> 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", \
> 
> This changed 0xcc to 0x90 but since it is just padding following an
> unconditional jmp so it shouldn't matter.

Well, it was for that very reason that I had picked 0xcc originally:
We better know if some branch mistakenly leads into that region.

I also very much object to the literal 2 passed as an argument to
.skip above: What if the label moves out far enough that a short
branch won't be usable anymore?

Jan


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