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Re: [Xen-devel] [PATCH] xen/arm: remove check for generic timer support for arm64

On Mon, 2014-06-02 at 12:13 +0100, Julien Grall wrote:
> On 06/02/2014 12:09 PM, Ian Campbell wrote:
> > On Mon, 2014-06-02 at 14:07 +0530, vijay.kilari@xxxxxxxxx wrote:
> >> From: Vijaya Kumar K <Vijaya.Kumar@xxxxxxxxxxxxxxxxxx>
> >>
> >> arm64 always supports generic timer. So check is not required
> >> for arm64. For platforms which supports only aarch64 mode this
> >> check always passes and panics
> > 
> > Ah, because the relevant feature flag/register is 32-bit only.
> > 
> > I'd prefer to see this done in the cpufeature header as:
> >         #ifdef CONFIG_ARM_32
> >         #define cpu_has_gentimer  (boot_cpu_feature32(gentimer) == 1)
> >         #else
> >         #define cpu_has_gentimer  (1)
> >         #endif
> > rather than adding #ifdef to the common code. Likewise for any similar
> > "always on for aarch64" features.
> AFAIU, the feature flag exists on ARMv8 platform with aarch32 support.
> So an ifdef may not be the correct solution here.

The flag might exist in the AArch32 feature registers (for compat with
v7) but AIUI the feature is not actually optional on v8.

Unless you think it is within the ARMv8 spec for a processor to
implement generic timers only when running in AArch64 mode and not
expose them to AArch32 mode? I don't think that is allowed.


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