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From: "Madhavan T. Venkataraman" <madvenka@linux.microsoft.com>
To: DJ Delorie <dj@redhat.com>
Cc: libffi-discuss@sourceware.org
Subject: Re: [RFC PATCH v3 2/5] x86: Support for Static Trampolines
Date: Thu, 28 Jan 2021 17:25:03 -0600	[thread overview]
Message-ID: <78a00ccd-3724-2a2d-3c59-f4a2b7a309cd@linux.microsoft.com> (raw)
In-Reply-To: <xnv9bgg0z0.fsf@greed.delorie.com>



On 1/28/21 4:17 PM, DJ Delorie wrote:
> "Madhavan T. Venkataraman" <madvenka@linux.microsoft.com> writes:
>>> Extern, but local to this port, yes?
>>
>> Yes. So, is this declaration acceptable?
> 
> Yup!

Great.

> 
>>>> +  /* Initialize the dynamic trampoline. */
>>>
>>> Should these new APIs be inside #if FFI_EXEC_STATIC_TRAMP ?
>>>
>>
>> Strictly speaking, these should be inside that ifdef. I did it this
>> way to avoid too many ifdefs in the code. If you prefer I put them
>> inside the ifdefs, I will do it. I will try to minimize the number of
>> ifdefs somehow.
> 
> No, it's ok, I was just worried that if ffi_tramp_is_present was in an
> #ifdef any callers would be too - but as you noted in another email,
> there's always a ffi_tramp_is_present even if it always returns false.
> 

ok.

>>> This hack to detect CET should be replaced by the logic in ffitarget.h,
>>> or add a #define CET_ENABLED to ffitarget.h
>>>
>>
>> So, _CET_ENDBR for x64 is either defined as:
>>
>> If CET is present:
>> 	#define _CET_ENDBR	endbr64
>> Otherwise:
>> 	#define _CET_ENDBR
>>
>> So, it is always defined. So, I cannot do something like:
> 
> I was thinking of the conditionals in src/x86/ffitarget.h:
> 
> #if !defined(GENERATE_LIBFFI_MAP) && defined(__ASSEMBLER__) \
>     && defined(__CET__)
> 
> Obviously you'd omit the __ASSEMBLER__ one ;-)

So, the definition is like this:

#if !defined(GENERATE_LIBFFI_MAP) && defined(__ASSEMBLER__) \
    && defined(__CET__)
# include <cet.h>
# define _CET_NOTRACK notrack
#else
# define _CET_ENDBR
# define _CET_NOTRACK
#endif

Where we include cet.h, CET_ENDBR may still be defined either
as endbr64 or empty depending on __CET__ & 1 being non-zero or
zero, right?

So, would something like this work?

#if !defined(GENERATE_LIBFFI_MAP) && defined(__CET__)
# include <cet.h>
# if (__CET__ & 1) != 0
#  define ENDBR_PRESENT
# endif
# define _CET_NOTRACK notrack
#else
# define _CET_ENDBR
# define _CET_NOTRACK
#endif

Then, I can do:

#ifdef ENDBR_PRESENT
blah blah

> 
> If you duplicate the ffitarget.h logic you get the same results.
> 
>>> Copies first argument to %r10, discards return address and arg - closure
>>> will return to whoever called it's caller.  I'm not sure how this works,
>>> which means *at least* a comment needs to be here ;-)
>>>
>> The target code in this case is the alt entry point.
>>
>> This is what the alt code is doing:
>> 	- load the data (closure) address in r10
>> 	- discard the original value of r10 saved on the stack
>> 	  since we are using r10 we don't need its original value
>> 	- restore the stack back to what it was when the static trampoline was
>> 	  invoked.
> 
> These kinds of short notes should be useful inline comments in the assembler:
> 
> 	movq	8(%rsp), %r10	/* load closure */
> 

I will add this.

>> I could add a small comment saying "see comment above trampoline_code_table".
>> Is that acceptable?
> 
> That would be good too :-)
> 

I will add this as well.

Thanks!

Madhavan

  reply	other threads:[~2021-01-28 23:25 UTC|newest]

Thread overview: 30+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <1ef5c7e1c9a6ebb140a476ba555ec955681f4fba>
2021-01-15 18:46 ` [RFC PATCH v3 0/5] Libffi " madvenka
2021-01-15 18:46   ` [RFC PATCH v3 1/5] " madvenka
2021-01-27  3:31     ` DJ Delorie
2021-01-27 21:51       ` Madhavan T. Venkataraman
2021-01-27 22:15         ` DJ Delorie
2021-01-27 22:43           ` Madhavan T. Venkataraman
2021-01-15 18:46   ` [RFC PATCH v3 2/5] x86: Support for " madvenka
2021-01-27  3:31     ` DJ Delorie
2021-01-28 21:59       ` Madhavan T. Venkataraman
2021-01-28 22:17         ` DJ Delorie
2021-01-28 23:25           ` Madhavan T. Venkataraman [this message]
2021-01-29  2:09             ` DJ Delorie
2021-01-29  2:38               ` Madhavan T. Venkataraman
2021-01-29  2:48                 ` DJ Delorie
2021-01-29  3:24                   ` Madhavan T. Venkataraman
2021-01-29  6:07                     ` DJ Delorie
2021-02-01 19:46                 ` DJ Delorie
2021-01-15 18:46   ` [RFC PATCH v3 3/5] i386: " madvenka
2021-01-15 18:46   ` [RFC PATCH v3 4/5] arm64: " madvenka
2021-01-15 18:46   ` [RFC PATCH v3 5/5] arm: " madvenka
2021-01-26 23:41   ` [RFC PATCH v3 0/5] Libffi " Anthony Green
2021-01-27 17:20     ` Madhavan T. Venkataraman
2021-01-27 18:00       ` Anthony Green
2021-01-27 19:45         ` Madhavan T. Venkataraman
2021-01-28 14:21           ` Anthony Green
2021-01-28 17:01             ` Madhavan T. Venkataraman
2021-02-05 18:20               ` Madhavan T. Venkataraman
2021-02-05 18:46                 ` Anthony Green
2021-02-05 19:38                   ` Madhavan T. Venkataraman
2021-02-07 16:09                     ` Madhavan T. Venkataraman

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