Re: [PATCH v5 02/27] arm64: cpufeature: Use alternatives for VHE cpu_enable

[Julien Thierry]

Hi James,

On 12/09/18 11:28, James Morse wrote:
> Hi Julien,
>
> On 28/08/18 16:51, Julien Thierry wrote:
> > The cpu_enable callback for VHE feature requires all alternatives to have
> > been applied. This prevents applying VHE alternative separately from the
> > rest.
> >
> > Use an alternative depending on VHE feature to know whether VHE
> > alternatives have already been applied.
>
> > diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> > index 1e433ac..3bc1c8b 100644
> > --- a/arch/arm64/kernel/cpufeature.c
> > +++ b/arch/arm64/kernel/cpufeature.c
> > @@ -1022,8 +1024,15 @@ static void cpu_copy_el2regs(const struct arm64_cpu_capabilities *__unused)
> >   	 * that, freshly-onlined CPUs will set tpidr_el2, so we don't need to
> >   	 * do anything here.
> >   	 */
> > -	if (!alternatives_applied)
> > -		write_sysreg(read_sysreg(tpidr_el1), tpidr_el2);
> > +	asm volatile(ALTERNATIVE(
> > +		"mrs	%0, tpidr_el1\n"
> > +		"msr	tpidr_el2, %0",
> > +		"nop\n"
> > +		"nop",
> > +		ARM64_HAS_VIRT_HOST_EXTN)
> > +		: "+r" (tmp)
> > +		:
> > +		: "memory");
> >   }
> >   #endif
>
> Catalin's preference was to keep this all in C:
> https://patchwork.kernel.org/patch/10007977/
> , for which we need to know if 'the' alternative has been applied.
>
> I suspect there may be more registers in this list if we have to switch to
> another EL2 register using alternatives. (but I don't have an example).
>
> Could we make 'alternatives_applied' a macro that takes the cap as an argument?

I wanted to do this initially, the issue was that the alternatives 
framework works on regions to patch rather than caps to apply. So I 
found it a bit odd to associate the "code corresponding to cap was 
applied" with the alternative application.

Although in patch 3 with the feature mask I guess that at the end of the 
__apply_alternatives loop, if the cap was part of the mask passed, it 
can be considered as applied.
If the asm inline is problematic I can go that route.

Thanks,

--
Julien Thierry