Re: [PATCH v2 14/16] arm64: Add a capability for FEAT_ECV

On Thu, Sep 30, 2021 at 08:42:56AM +0100, Marc Zyngier wrote:

On Wed, 29 Sep 2021 17:03:30 +0100,
Will Deacon [off-list ref] wrote:

quoted

On Wed, Sep 22, 2021 at 10:19:39PM +0100, Marc Zyngier wrote:

quoted

Add a new capability to detect the Enhanced Counter Virtualization
feature (FEAT_ECV).

Reviewed-by: Oliver Upton <redacted>
Signed-off-by: Marc Zyngier <maz@kernel.org>
---
 arch/arm64/kernel/cpufeature.c | 10 ++++++++++
 arch/arm64/tools/cpucaps       |  1 +
 2 files changed, 11 insertions(+)

diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index f8a3067d10c6..26b11ce8fff6 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c

@@ -1926,6 +1926,16 @@ static const struct arm64_cpu_capabilities arm64_features[] = {
 		.sign = FTR_UNSIGNED,
 		.min_field_value = 1,
 	},
+	{
+		.desc = "Enhanced Counter Virtualization",
+		.capability = ARM64_HAS_ECV,
+		.type = ARM64_CPUCAP_SYSTEM_FEATURE,
+		.matches = has_cpuid_feature,
+		.sys_reg = SYS_ID_AA64MMFR0_EL1,
+		.field_pos = ID_AA64MMFR0_ECV_SHIFT,
+		.sign = FTR_UNSIGNED,
+		.min_field_value = 1,
+	},

Could we add a HWCAP for this and change the field to FTR_VISIBLE, please? I
know most users of the counter are indirected via the vDSO, but there are
some users out there using the counter directly and it would save them
having to probe via SIGILL if there was a hwcap available.

Fair enough, I'll add that.

Thanks!

The problem of the vdso remains though, and is by far the most common
user of the feature. Any idea on how to handle it? Patching the vdso
is ugly, and I'd rather avoid it.

I briefly looked at using ifunc, but it is likely that the indirection
would add an extra cost. Are we OK with that?

The vDSO is still miles faster than a system call, so I'd be inclined to
leave it as-is for the time being. I suspect that use-cases which can't
stomach the cost of the ISB can't stomach the cost of the vDSO at all.

Will

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