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

Hi James,

On 12/09/18 11:28, James Morse wrote:

Hi Julien,

On 28/08/18 16:51, Julien Thierry wrote:

quoted

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.

quoted

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