linux-kernel - Re: [PATCH v3 11/13] KVM: selftests: x86: Remove redundant check that XSAVE is supported

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Message-ID: <ZB4LfyVjDuncOsM7@google.com>
Date:   Fri, 24 Mar 2023 13:43:43 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Mingwei Zhang <mizhang@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, linux-kselftest@...r.kernel.org,
        Jim Mattson <jmattson@...gle.com>,
        Venkatesh Srinivas <venkateshs@...gle.com>,
        Aaron Lewis <aaronlewis@...gle.com>,
        "Chang S. Bae" <chang.seok.bae@...el.com>,
        Chao Gao <chao.gao@...el.com>
Subject: Re: [PATCH v3 11/13] KVM: selftests: x86: Remove redundant check that
 XSAVE is supported

On Tue, Feb 21, 2023, Mingwei Zhang wrote:
> From: Aaron Lewis <aaronlewis@...gle.com>
> 
> In amx_test, userspace requires that XSAVE is supported before running
> the test, then the guest checks that it is supported after enabling
> AMX.  Remove the redundant check in the guest that XSAVE is supported.

It's a bit paranoid, but I actually don't mind the extra check.  It's not redundant
per se, just useless in its current location.  If the check is moved _before_
CR4 is set, then it actually provides value, e.g. if something does go sideways,
will fire an assert instead of getting a #GP on set_cr4().