Re: [PATCH] arm64: cpufeature: Don't expose ZFR0 to userspace when SVE is not enabled

From: Suzuki K Poulose
Date: Mon Oct 14 2019 - 12:57:50 EST

On 14/10/2019 17:43, Will Deacon wrote:
On Mon, Oct 14, 2019 at 11:21:13AM +0100, Julien Grall wrote:
The kernel may not support SVE if CONFIG_ARM64_SVE is not set and
will hide the feature from the from userspace.

I don't understand this sentence.

Unfortunately, the fields of ID_AA64ZFR0_EL1 are still exposed and could
lead to undefined behavior in userspace.

Undefined in what way? Generally, we can't stop exposing things that
we've exposed previously in case somebody has started relying on them, so
this needs better justification.

We still expose them with this patch, but zero them out, if the SVE is not
supported. When SVE is enabled, we expose them as usual.