Re: [RFC PATCH v2 3/7] arm64: alternative: Apply alternatives early in boot process

[Will Deacon]

On Thu, Sep 17, 2015 at 02:25:56PM +0100, Daniel Thompson wrote:
> On 16/09/15 17:24, Will Deacon wrote:
> > 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:
> >>>>    /*
> >>>> + * 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(&region);
> >>>> +}
> >>>
> >>> 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?
> 
> Hmmnn... I don't think this patch will impact that.
> 
> Once something in the boot sequence calls cpus_set_cap() then if there 
> is a corresponding alternative then it is *going* to be applied isn't 
> it? The patch only means that some of the alternatives will be applied 
> early. Once the boot is complete the patched .text should be the same 
> with and without the patch.
> 
> Have I overlooked some code in the current kernel that prevents a system 
> with mis-matched LSE support from applying the alternatives?

Sorry, I'm thinking slightly ahead of myself, but the series from Suzuki
creates a shadow "safe" view of the ID registers in the system,
corresponding to the intersection of CPU features:

  http://lists.infradead.org/pipermail/linux-arm-kernel/2015-September/370386.html

In this case, it is necessary to inspect all of the possible CPUs before
we can apply the patching, but as I say above, I'm prepared to make an
exception for NMI because I don't think we can assume a safe value anyway
for a system with mismatched GIC CPU interfaces. I just don't want to
drag all of the alternatives patching earlier as well.

> > 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.
> 
> Good point. I'll fix this in the next respin.

Great, thanks. It probably also means that the NMI code needs
__kprobes/__notrace annotations for similar reasons.

Will
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