From: Will Deacon <will.deacon@arm.com>
To: Daniel Thompson <daniel.thompson@linaro.org>
Cc: "linux-arm-kernel@lists.infradead.org"
<linux-arm-kernel@lists.infradead.org>,
Catalin Marinas <Catalin.Marinas@arm.com>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
"patches@linaro.org" <patches@linaro.org>,
"linaro-kernel@lists.linaro.org" <linaro-kernel@lists.linaro.org>,
John Stultz <john.stultz@linaro.org>,
Sumit Semwal <sumit.semwal@linaro.org>,
Marc Zyngier <Marc.Zyngier@arm.com>,
Andrew Thoelke <Andrew.Thoelke@arm.com>,
Dave P Martin <Dave.Martin@arm.com>
Subject: Re: [RFC PATCH v2 3/7] arm64: alternative: Apply alternatives early in boot process
Date: Wed, 16 Sep 2015 17:24:52 +0100 [thread overview]
Message-ID: <20150916162452.GN28771@arm.com> (raw)
In-Reply-To: <55F98FF0.7030605@linaro.org>
On Wed, Sep 16, 2015 at 04:51:12PM +0100, Daniel Thompson wrote:
> On 16/09/15 14:05, Will Deacon wrote:
> > On Mon, Sep 14, 2015 at 02:26:17PM +0100, Daniel Thompson wrote:
> >> Currently alternatives are applied very late in the boot process (and
> >> a long time after we enable scheduling). Some alternative sequences,
> >> such as those that alter the way CPU context is stored, must be applied
> >> much earlier in the boot sequence.
> >>
> >> Introduce apply_alternatives_early() to allow some alternatives to be
> >> applied immediately after we detect the CPU features of the boot CPU.
> >>
> >> Currently apply_alternatives_all() is not optimized and will re-patch
> >> code that has already been updated. This is harmless but could be
> >> removed by adding extra flags to the alternatives store.
> >>
> >> Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>
> >> ---
> > [snip]
> >> /*
> >> + * This is called very early in the boot process (directly after we run
> >> + * a feature detect on the boot CPU). No need to worry about other CPUs
> >> + * here.
> >> + */
> >> +void apply_alternatives_early(void)
> >> +{
> >> + struct alt_region region = {
> >> + .begin = __alt_instructions,
> >> + .end = __alt_instructions_end,
> >> + };
> >> +
> >> + __apply_alternatives(®ion);
> >> +}
> >
> > How do you choose which alternatives are applied early and which are
> > applied later? AFAICT, this just applies everything before we've
> > established the capabilities of the CPUs in the system, which could cause
> > problems for big/little SoCs.
>
> They are applied twice. This relies for correctness on the fact that
> cpufeatures can be set but not unset.
>
> In other words the boot CPU does a feature detect and, as a result, a
> subset of the required alternatives will be applied. However after this
> the other CPUs will boot and the the remaining alternatives applied as
> before.
>
> The current implementation is inefficient (because it will redundantly
> patch the same code twice) but I don't think it is broken.
What about a big/little system where we boot on the big cores and only
they support LSE atomics?
> > Also, why do we need this for the NMI?
>
> I was/am concerned that a context saved before the alternatives are
> applied might be restored afterwards. If that happens the bit that
> indicates what value to put into the PMR would read during the restore
> without having been saved first. Applying early ensures that the context
> save/restore code is updated before it is ever used.
Damn, and stop_machine makes use of local_irq_restore immediately after
the patching has completed, so it's a non-starter. Still, special-casing
this feature via an explicit apply_alternatives call would be better
than moving everything earlier, I think.
We also need to think about how an incoming NMI interacts with
concurrent patching of later features. I suspect we want to set the I
bit, like you do for WFI, unless you can guarantee that no patched
sequences run in NMI context.
Will
WARNING: multiple messages have this Message-ID (diff)
From: will.deacon@arm.com (Will Deacon)
To: linux-arm-kernel@lists.infradead.org
Subject: [RFC PATCH v2 3/7] arm64: alternative: Apply alternatives early in boot process
Date: Wed, 16 Sep 2015 17:24:52 +0100 [thread overview]
Message-ID: <20150916162452.GN28771@arm.com> (raw)
In-Reply-To: <55F98FF0.7030605@linaro.org>
On Wed, Sep 16, 2015 at 04:51:12PM +0100, Daniel Thompson wrote:
> On 16/09/15 14:05, Will Deacon wrote:
> > On Mon, Sep 14, 2015 at 02:26:17PM +0100, Daniel Thompson wrote:
> >> Currently alternatives are applied very late in the boot process (and
> >> a long time after we enable scheduling). Some alternative sequences,
> >> such as those that alter the way CPU context is stored, must be applied
> >> much earlier in the boot sequence.
> >>
> >> Introduce apply_alternatives_early() to allow some alternatives to be
> >> applied immediately after we detect the CPU features of the boot CPU.
> >>
> >> Currently apply_alternatives_all() is not optimized and will re-patch
> >> code that has already been updated. This is harmless but could be
> >> removed by adding extra flags to the alternatives store.
> >>
> >> Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>
> >> ---
> > [snip]
> >> /*
> >> + * This is called very early in the boot process (directly after we run
> >> + * a feature detect on the boot CPU). No need to worry about other CPUs
> >> + * here.
> >> + */
> >> +void apply_alternatives_early(void)
> >> +{
> >> + struct alt_region region = {
> >> + .begin = __alt_instructions,
> >> + .end = __alt_instructions_end,
> >> + };
> >> +
> >> + __apply_alternatives(®ion);
> >> +}
> >
> > How do you choose which alternatives are applied early and which are
> > applied later? AFAICT, this just applies everything before we've
> > established the capabilities of the CPUs in the system, which could cause
> > problems for big/little SoCs.
>
> They are applied twice. This relies for correctness on the fact that
> cpufeatures can be set but not unset.
>
> In other words the boot CPU does a feature detect and, as a result, a
> subset of the required alternatives will be applied. However after this
> the other CPUs will boot and the the remaining alternatives applied as
> before.
>
> The current implementation is inefficient (because it will redundantly
> patch the same code twice) but I don't think it is broken.
What about a big/little system where we boot on the big cores and only
they support LSE atomics?
> > Also, why do we need this for the NMI?
>
> I was/am concerned that a context saved before the alternatives are
> applied might be restored afterwards. If that happens the bit that
> indicates what value to put into the PMR would read during the restore
> without having been saved first. Applying early ensures that the context
> save/restore code is updated before it is ever used.
Damn, and stop_machine makes use of local_irq_restore immediately after
the patching has completed, so it's a non-starter. Still, special-casing
this feature via an explicit apply_alternatives call would be better
than moving everything earlier, I think.
We also need to think about how an incoming NMI interacts with
concurrent patching of later features. I suspect we want to set the I
bit, like you do for WFI, unless you can guarantee that no patched
sequences run in NMI context.
Will
next prev parent reply other threads:[~2015-09-16 16:24 UTC|newest]
Thread overview: 35+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-09-14 13:26 [RFC PATCH v2 0/7] Pseudo-NMI for arm64 using ICC_PMR_EL1 (GICv3) Daniel Thompson
2015-09-14 13:26 ` Daniel Thompson
2015-09-14 13:26 ` [RFC PATCH v2 1/7] irqchip: gic-v3: Reset BPR during initialization Daniel Thompson
2015-09-14 13:26 ` Daniel Thompson
2015-09-14 13:26 ` [RFC PATCH v2 2/7] arm64: Add support for on-demand backtrace of other CPUs Daniel Thompson
2015-09-14 13:26 ` Daniel Thompson
2015-09-14 13:26 ` [RFC PATCH v2 3/7] arm64: alternative: Apply alternatives early in boot process Daniel Thompson
2015-09-14 13:26 ` Daniel Thompson
2015-09-16 13:05 ` Will Deacon
2015-09-16 13:05 ` Will Deacon
2015-09-16 15:51 ` Daniel Thompson
2015-09-16 15:51 ` Daniel Thompson
2015-09-16 16:24 ` Will Deacon [this message]
2015-09-16 16:24 ` Will Deacon
2015-09-17 13:25 ` Daniel Thompson
2015-09-17 13:25 ` Daniel Thompson
2015-09-17 14:01 ` Will Deacon
2015-09-17 14:01 ` Will Deacon
2015-09-17 15:28 ` Daniel Thompson
2015-09-17 15:28 ` Daniel Thompson
2015-09-17 15:43 ` Will Deacon
2015-09-17 15:43 ` Will Deacon
2015-09-14 13:26 ` [RFC PATCH v2 4/7] arm64: irqflags: Reorder the fiq & async macros Daniel Thompson
2015-09-14 13:26 ` Daniel Thompson
2015-09-14 13:26 ` [RFC PATCH v2 5/7] arm64: irqflags: Use ICC sysregs to implement IRQ masking Daniel Thompson
2015-09-14 13:26 ` Daniel Thompson
2015-09-14 13:26 ` [RFC PATCH v2 6/7] arm64: Implement IPI_CPU_BACKTRACE using pseudo-NMIs Daniel Thompson
2015-09-14 13:26 ` Daniel Thompson
2015-09-14 13:26 ` [RFC PATCH v2 7/7] arm64: irqflags: Automatically identify I bit mis-management Daniel Thompson
2015-09-14 13:26 ` Daniel Thompson
2015-09-18 5:11 ` [RFC PATCH v2 0/7] Pseudo-NMI for arm64 using ICC_PMR_EL1 (GICv3) Jon Masters
2015-09-18 5:11 ` Jon Masters
2015-09-18 11:23 ` Daniel Thompson
2015-09-18 11:23 ` Daniel Thompson
2015-09-22 18:08 ` 答复: " Dingtianhong
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