From mboxrd@z Thu Jan 1 00:00:00 1970 From: Vincenzo Frascino Subject: Re: [PATCH 1/5] arm64: Fix vDSO clock_getres() Date: Tue, 16 Apr 2019 13:42:58 +0100 Message-ID: References: <20190401115152.32751-1-vincenzo.frascino@arm.com> <20190401115152.32751-2-vincenzo.frascino@arm.com> <20190415173521.GA901@arrakis.emea.arm.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20190415173521.GA901@arrakis.emea.arm.com> Content-Language: en-US List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=m.gmane.org@lists.infradead.org List-Archive: List-Post: To: Catalin Marinas Cc: linux-arch@vger.kernel.org, linux-s390@vger.kernel.org, Shuah Khan , Arnd Bergmann , Heiko Carstens , Benjamin Herrenschmidt , Will Deacon , Paul Mackerras , Greentime Hu , Michael Ellerman , Martin Schwidefsky , Thomas Gleixner , Vincent Chen , linuxppc-dev@lists.ozlabs.org, linux-arm-kernel@lists.infradead.org List-ID: Hi Catalin, On 15/04/2019 18:35, Catalin Marinas wrote: > On Mon, Apr 01, 2019 at 12:51:48PM +0100, Vincenzo Frascino wrote: >> diff --git a/arch/arm64/kernel/vdso.c b/arch/arm64/kernel/vdso.c >> index 2d419006ad43..47ba72345739 100644 >> --- a/arch/arm64/kernel/vdso.c >> +++ b/arch/arm64/kernel/vdso.c >> @@ -245,6 +245,8 @@ void update_vsyscall(struct timekeeper *tk) >> vdso_data->cs_shift = tk->tkr_mono.shift; >> } >> >> + vdso_data->hrtimer_res = hrtimer_resolution; >> + >> smp_wmb(); >> ++vdso_data->tb_seq_count; >> } >> diff --git a/arch/arm64/kernel/vdso/gettimeofday.S b/arch/arm64/kernel/vdso/gettimeofday.S >> index c39872a7b03c..7a2cd2f8e13a 100644 >> --- a/arch/arm64/kernel/vdso/gettimeofday.S >> +++ b/arch/arm64/kernel/vdso/gettimeofday.S >> @@ -296,32 +296,35 @@ ENDPROC(__kernel_clock_gettime) >> /* int __kernel_clock_getres(clockid_t clock_id, struct timespec *res); */ >> ENTRY(__kernel_clock_getres) >> .cfi_startproc >> + adr vdso_data, _vdso_data >> cmp w0, #CLOCK_REALTIME >> ccmp w0, #CLOCK_MONOTONIC, #0x4, ne >> ccmp w0, #CLOCK_MONOTONIC_RAW, #0x4, ne >> - b.ne 1f >> + b.ne 2f >> >> - ldr x2, 5f >> - b 2f >> -1: >> +1: /* Get hrtimer_res */ >> + seqcnt_acquire >> + syscall_check fail=5f >> + ldr x2, [vdso_data, #CLOCK_REALTIME_RES] >> + seqcnt_check fail=1b >> + b 3f >> +2: > > We talked briefly but I'm still confused why we need the fallback to the > syscall here if archdata.vdso_direct is false. Is it because if the > timer driver code sets vdso_direct to false, we don't don't support > highres timers? If my understanding is correct, you may want to move the > hrtimer_res setting in update_vsyscall() to the !use_syscall block. > Ok, so let me try to provide more details on what I mentioned yesterday: - clock_getres syscall follows the rules of what defined in posix-timers.c - based on the clock_id that, for this purpose, can be separated in coarse and non-coarse calls either posix_get_coarse_res() or posix_get_hrtimer_res(). - if clock id is set to a coarse clock and posix_get_coarse_res() is invoked, happens what follows: static int posix_get_coarse_res(const clockid_t which_clock, struct timespec64 *tp) { *tp = ktime_to_timespec64(KTIME_LOW_RES); return 0; } Note that since CONFIG_1HZ seems not supported (jiffies.h) by the kernel in this case we do not need rounding in our vDSO implementation. - if clock id is set to non-coarse and posix_get_hrtimer_res() is invoked, happens the following: static int posix_get_hrtimer_res(clockid_t which_clock, struct timespec64 *tp) { tp->tv_sec = 0; tp->tv_nsec = hrtimer_resolution; return 0; } hrtimer_resolution can be high res or low res depending on the call of hrtimer_switch_to_hres(). For us the only way to preserve the correct value is to keep it in the vdso data page. - The assembly code mimics exactly the same behaviour detailed above, with one difference: the one related to the use_syscall parameter which is specific to arm64. The use_syscall parameter is set by arm_arch_timer and consumed by update_vsyscall(). To mirror what update_vsyscall does in update_vsyscall() I check "syscall_check fail=5f" in clock_getres vdso function. Said that, even if functionally it is the same thing, I think it is logically more correct to have hrtimer_res setting inside the !use_syscall block, hence I am going to change it in the next iteration. Please let me know your thoughts. -- Regards, Vincenzo