Re: [Xen-devel] [PATCH v8 01/17] x86: Support compiling with indirect branch thunks

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 15.01.18 at 11:40, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 15/01/18 10:14, Jan Beulich wrote:
>>>>> On 12.01.18 at 19:00, <andrew.cooper3@xxxxxxxxxx> wrote:
>>> --- a/xen/arch/x86/Rules.mk
>>> +++ b/xen/arch/x86/Rules.mk
>>> @@ -30,3 +30,10 @@ CFLAGS += -fno-asynchronous-unwind-tables
>>>  ifneq ($(call cc-option,$(CC),-fvisibility=hidden,n),n)
>>>  CFLAGS += -DGCC_HAS_VISIBILITY_ATTRIBUTE
>>>  endif
>>> +
>>> +# Compile with thunk-extern, indirect-branch-register if avaiable.
>>> +ifneq ($(call cc-option,$(CC),-mindirect-branch-register,n),n)
>>> +CFLAGS += -mindirect-branch=thunk-extern -mindirect-branch-register
>>> +CFLAGS += -DCONFIG_INDIRECT_THUNK
>>> +export CONFIG_INDIRECT_THUNK=y
>>> +endif
>> Still not a proper config option?
> 
> No, for backportability.
> 
> I'm preparing a post-SP2 series with some cleanup for staging.
> 
>>
>>> --- /dev/null
>>> +++ b/xen/arch/x86/indirect-thunk.S
>>> @@ -0,0 +1,38 @@
>>> +/*
>>> + * Implement __x86_indirect_thunk_* symbols for use with compatbile 
> compilers
>>> + * and the -mindirect-branch=thunk-extern -mindirect-branch-register 
> options.
>>> + *
>>> + * Copyright (c) 2017-2018 Citrix Systems Ltd.
>>> + *
>>> + * This source code is licensed under the GNU General Public License,
>>> + * Version 2.  See the file COPYING for more details.
>>> + */
>>> +        .file __FILE__
>>> +
>>> +#include <asm/asm_defns.h>
>>> +
>>> +.macro IND_THUNK_RETPOLINE reg:req
>>> +        call 2f
>>> +1:
>>> +        lfence
>>> +        jmp 1b
>>> +2:
>> As noted in a couple of other places, I'd prefer if numeric labels
>> weren't used in macros (and especially new ones), in favor of
>> .L ones utilizing \@.
> 
> For macros in header files, I can see this rational.
> 
> However, this is a local macro which only gets expanded in this file,
> and isn't likely to move elsewhere.  I don't see the value in reducing
> the readability.
> 
>>
>>> +        mov %\reg, (%rsp)
>>> +        ret
>>> +.endm
>>> +
>>> +/*
>>> + * Build the __x86_indirect_thunk_* symbols.  Currently implement the
>>> + * retpoline thunk only.
>>> + */
>>> +.macro GEN_INDIRECT_THUNK reg:req
>>> +        .section .text.__x86_indirect_thunk_\reg, "ax", @progbits
>>> +
>>> +ENTRY(__x86_indirect_thunk_\reg)
>>> +        IND_THUNK_RETPOLINE \reg
>>> +.endm
>> Still unnecessary leading underscores in the section name?
> 
> Having the section names different to the symbols in them is far worse
> than using double underscores.

Well, okay then:
Acked-by: Jan Beulich <jbeulich@xxxxxxxx>

Jan


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