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

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Message-ID: <20191014164313.hu2dnf5rokntzhhp@willie-the-truck>
Date:   Mon, 14 Oct 2019 17:43:14 +0100
From:   Will Deacon <will@...nel.org>
To:     Julien Grall <julien.grall@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        catalin.marinas@....com, suzuki.poulose@....com,
        Dave.Martin@....com
Subject: Re: [PATCH] arm64: cpufeature: Don't expose ZFR0 to userspace when
 SVE is not enabled

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.

Will