[PATCH 3/4] arm64: add ARM64_HAS_GIC_PRIO_NO_PMHE cpucap

[Marc Zyngier]

On Mon, 23 Jan 2023 12:40:41 +0000,
Mark Rutland <mark.rutland at arm.com> wrote:
> 
> When Priority Mask Hint Enable (PMHE) == 0b1, the GIC may use the PMR
> value to determine whether to signal an IRQ to a PE, and consequently
> after a change to the PMR value, a DSB SY may be required to ensure that
> interrupts are signalled to a CPU in finite time. When PMHE == 0b0,
> interrupts are always signalled to the relevant PE, and all masking
> occurs locally, without requiring a DSB SY.
> 
> Since commit:
> 
>   f226650494c6aa87 ("arm64: Relax ICC_PMR_EL1 accesses when ICC_CTLR_EL1.PMHE is clear")
> 
> ... we handle this dynamically: in most cases a static key is used to
> determine whether to issue a DSB SY, but the entry code must read from
> ICC_CTLR_EL1 as static keys aren't accessible from plain assembly.
> 
> It would be much nicer to use an alternative instruction sequence for
> the DSB, as this would avoid the need to read from ICC_CTLR_EL1 in the
> entry code, and for most other code this will result in simpler code
> generation with fewer instructions and fewer branches.
> 
> This patch adds a new ARM64_HAS_GIC_PRIO_NO_PMHE cpucap which is only
> set when ICC_CTLR_EL1.PMHE == 0b0 (and GIC priority masking is in use).
> This allows us to replace the existing users of the `gic_pmr_sync`
> static key with alternative sequences which default to a DSB SY and are
> relaxed to a NOP when PMHE is not in use.

I personally find the "negative capability" pretty annoying, specially
considering that hardly anyone uses PMHE. The way the code reads with
this patch, it is always some sort of double negation.

Can't the DSB be patched-in instead, making the PMHE cap a "positive"
one? It shouldn't affect interrupt distribution as long as the
patching occurs before we take interrupts. For modules, the patching
always occurs before we can run the module, so this should be equally
safe.

The patch otherwise looks OK to me.

	M.

-- 
Without deviation from the norm, progress is not possible.