Message-ID: <b4563946-9653-4546-b52d-4a9299ad63da@kernel.org>
Date: Thu, 22 Jan 2026 14:30:55 +0100
From: "Christophe Leroy (CS GROUP)" <chleroy@...nel.org>
To: Thomas Weißschuh <thomas.weissschuh@...utronix.de>
Cc: "Sverdlin, Alexander" <alexander.sverdlin@...mens.com>,
 "npiggin@...il.com" <npiggin@...il.com>, "luto@...nel.org"
 <luto@...nel.org>, "maddy@...ux.ibm.com" <maddy@...ux.ibm.com>,
 "tglx@...nel.org" <tglx@...nel.org>, "mpe@...erman.id.au"
 <mpe@...erman.id.au>, "vincenzo.frascino@....com"
 <vincenzo.frascino@....com>,
 "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] powerpc/vdso: Provide clock_getres_time64()



Le 22/01/2026 à 12:41, Thomas Weißschuh a écrit :
> On Thu, Jan 22, 2026 at 12:31:32PM +0100, Christophe Leroy (CS GROUP) wrote:
>> Le 22/01/2026 à 12:07, Thomas Weißschuh a écrit :
>>> On Thu, Jan 22, 2026 at 11:58:04AM +0100, Christophe Leroy (CS GROUP) wrote:
>>>>
>>>>
>>>> Le 22/01/2026 à 11:49, Thomas Weißschuh a écrit :
>>>>> On Thu, Jan 22, 2026 at 11:27:43AM +0100, Christophe Leroy (CS GROUP) wrote:
>>>>>> Hi Thomas,
>>>>>>
>>>>>> Le 22/01/2026 à 10:50, Thomas Weißschuh a écrit :
>>>>>>> Hi Alexander,
>>>>>>>
>>>>>>> On Thu, Jan 22, 2026 at 09:39:09AM +0000, Sverdlin, Alexander wrote:
>>>>>>>> Hi Thomas, Christophe,
>>>>>>>>
>>>>>>>> On Wed, 2026-01-14 at 08:26 +0100, Thomas Weißschuh wrote:
>>>>>>>>> For consistency with __vdso_clock_gettime64() there should also be a
>>>>>>>>> 64-bit variant of clock_getres(). This will allow the extension of
>>>>>>>>> CONFIG_COMPAT_32BIT_TIME to the vDSO and finally the removal of 32-bit
>>>>>>>>> time types from the kernel and UAPI.
>>>>>>>>>
>>>>>>>>> Signed-off-by: Thomas Weißschuh <thomas.weissschuh@...utronix.de>
>>>>>>>>
>>>>>>>> I've bisected this patch to cause the following build failure on my side:
>>>>>>>>
>>>>>>>>       LDS     arch/powerpc/kernel/vdso/vdso32.lds
>>>>>>>>       VDSO32A arch/powerpc/kernel/vdso/sigtramp32-32.o
>>>>>>>>       VDSO32A arch/powerpc/kernel/vdso/gettimeofday-32.o
>>>>>>>>       VDSO32A arch/powerpc/kernel/vdso/datapage-32.o
>>>>>>>>       VDSO32A arch/powerpc/kernel/vdso/cacheflush-32.o
>>>>>>>>       VDSO32A arch/powerpc/kernel/vdso/note-32.o
>>>>>>>>       VDSO32A arch/powerpc/kernel/vdso/getcpu-32.o
>>>>>>>>       VDSO32A arch/powerpc/kernel/vdso/getrandom-32.o
>>>>>>>>       VDSO32A arch/powerpc/kernel/vdso/vgetrandom-chacha-32.o
>>>>>>>>       VDSO32C arch/powerpc/kernel/vdso/vgettimeofday-32.o
>>>>>>>>       VDSO32C arch/powerpc/kernel/vdso/vgetrandom-32.o
>>>>>>>>       VDSO32A arch/powerpc/kernel/vdso/crtsavres-32.o
>>>>>>>>       VDSO32L arch/powerpc/kernel/vdso/vdso32.so.dbg
>>>>>>>> arch/powerpc/kernel/vdso/vdso32.so.dbg: dynamic relocations are not supported
>>>>>>>> make[2]: *** [arch/powerpc/kernel/vdso/Makefile:79: arch/powerpc/kernel/vdso/vdso32.so.dbg] Error 1
>>>>>>>> make[1]: *** [arch/powerpc/Makefile:388: vdso_prepare] Error 2
>>>>>>>
>>>>>>> Thanks for the report!
>>>>>>>
>>>>>>>> Does it ring any bells? What could I try/test?
>>>>>>>
>>>>>>> Not immediately, but I'll look into it.
>>>>>>>
>>>>>>>> I'm using gcc-15.2.0 and binutils 2.45.1.
>>>>>>>
>>>>>>> Is this a toolchain from https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fcdn.kernel.org%2Fpub%2Ftools%2Fcrosstool%2F&data=05%7C02%7Cchristophe.leroy%40csgroup.eu%7Cbd6434eab4334cea44fe08de59ab274e%7C8b87af7d86474dc78df45f69a2011bb5%7C0%7C0%7C639046788822130060%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C%7C&sdata=K7PlfBLHdNEBRgsPQ5aNBgu2v8DqDrhmzwoHrcOc5s8%3D&reserved=0 ?
>>>>>>> Could you also share your configuration?
>>>>>>
>>>>>> I've just been able to reproduce it with ppc64_defconfig +
>>>>>> CONFIG_CC_OPTIMIZE_FOR_SIZE
>>>>>
>>>>> Thanks for the hint, no I can reproduce it, too.
>>>>>>
>>>>>>      VDSO32L arch/powerpc/kernel/vdso/vdso32.so.dbg
>>>>>> arch/powerpc/kernel/vdso/vdso32.so.dbg: dynamic relocations are not
>>>>>> supported
>>>>>> make[2]: *** [arch/powerpc/kernel/vdso/Makefile:79:
>>>>>> arch/powerpc/kernel/vdso/vdso32.so.dbg] Error 1
>>>>>> make[1]: *** [arch/powerpc/Makefile:388: vdso_prepare] Error 2
>>>>>> make: *** [Makefile:248: __sub-make] Error 2
>>>>>>
>>>>>> I'll investigate
>>>>>
>>>>> It seems the compiler decides to call memset(), which is not valid from the
>>>>> vDSO. We are are using -ffreestanding. Disabling CONFIG_INIT_STACK_ALL_ZERO
>>>>> fixes the issue. So I guess we should a) figure out why -ffreestanding does
>>>>> not seem to work here and b) exclude the vDSO from the stack initialization
>>>>> logic.
>>>>>
>>>>
>>>> Ah, ok.
>>>>
>>>> Reminds me commit b91c8c42ffdd ("lib/vdso: Force inlining of
>>>> __cvdso_clock_gettime_common()")
>>>
>>> Good pointer.
>>>
>>>> Problem fixed with:
>>>>
>>>> diff --git a/lib/vdso/gettimeofday.c b/lib/vdso/gettimeofday.c
>>>> index 95df0153f05ab..4399e143d43a5 100644
>>>> --- a/lib/vdso/gettimeofday.c
>>>> +++ b/lib/vdso/gettimeofday.c
>>>> @@ -421,7 +421,7 @@ static __maybe_unused __kernel_old_time_t
>>>> __cvdso_time(__kernel_old_time_t *time
>>>>    #endif /* VDSO_HAS_TIME */
>>>>
>>>>    #ifdef VDSO_HAS_CLOCK_GETRES
>>>> -static __maybe_unused
>>>> +static __always_inline
>>>>    bool __cvdso_clock_getres_common(const struct vdso_time_data *vd, clockid_t
>>>> clock,
>>>>    				 struct __kernel_timespec *res)
>>>>    {
>>>
>>> Do you want to run the measurements for this one, too and submit a fix?
>>> This should get us past the immediate breakage.
>>
>> I'm travelling at the moment and won't be able to come with measurement
>> before next month. But the performance degradation is obvious.
> 
> Ack, then I'll send a patch. Thanks for all the information.
> 
>> With the fix, the function is stackless:
> 
> (...)
> 
>> Without the fix, see below, __c_kernel_clock_getres() has to setup a stack
>> in order to call __cvdso_clock_getres_common(), and in addition we see that
>> __cvdso_clock_getres_common() is more or less the same size as
>> __c_kernel_clock_getres() above, so time increase unquestionable.
> 
> (...)
> 
>>> I'll still try to get the stack initialization out of the vDSO.
>>> It might bite us at any time in the future. As these options are meant
>>> to prevent information leaks and the vDSO has no sensitive information in
>>> the first place, we might as well filter them out.
>>
>> Well, from the first day we converted powerpc to C time vdso, we've done our
>> best in order to keep vdso stackless. So I'm not sure it is worth dealing
>> with the above. Indeed if keeping it as is helps us detect everytime a
>> change jeoperdises the stackless approach, that's not bad.
> 
> I was not aware about the stacklessness. Then this should be reason enough.
> We should get a better system to detect these additional stacks though.
> I'll think about it a bit more.

Well, that's only a "best effort", at the end we don't success in all 
cases, refer commit 08c18b63d965 ("powerpc/vdso32: Add missing 
_restgpr_31_x to fix build failure"), there are not enough volatile 
registers for __c_kernel_clock_gettime(), part of this function sets up 
a stack frame to save one non-volatile register and use it. But at least 
for time we can keep flat functions that don't have to suffer the 
overcost of saving volatile registers for calling subfunctions.

For __c_kernel_getrandom() that's different because it has to call 
__arch_chacha20_blocks_nostack() so a stack frame is definitely needed 
but at least we also avoid there to have multiple sub-function calls.

Christophe

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