linux-kernel - Re: [PATCH] x86/tsx: fix KVM guest live migration for tsx=on

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Date:   Mon, 11 Apr 2022 12:26:52 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Jon Kohler <jon@...anix.com>, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, Tony Luck <tony.luck@...el.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        linux-kernel@...r.kernel.org
Cc:     Borislav Petkov <bp@...e.de>,
        Neelima Krishnan <neelima.krishnan@...el.com>,
        "kvm @ vger . kernel . org" <kvm@...r.kernel.org>
Subject: Re: [PATCH] x86/tsx: fix KVM guest live migration for tsx=on

On 4/11/22 11:01, Jon Kohler wrote:
>  static enum tsx_ctrl_states x86_get_tsx_auto_mode(void)
>  {
> +	/*
> +	 * Hardware will always abort a TSX transaction if both CPUID bits
> +	 * RTM_ALWAYS_ABORT and TSX_FORCE_ABORT are set. In this case, it is
> +	 * better not to enumerate CPUID.RTM and CPUID.HLE bits. Clear them
> +	 * here.
> +	 */
> +	if (boot_cpu_has(X86_FEATURE_RTM_ALWAYS_ABORT) &&
> +	    boot_cpu_has(X86_FEATURE_TSX_FORCE_ABORT)) {
> +		tsx_clear_cpuid();
> +		setup_clear_cpu_cap(X86_FEATURE_RTM);
> +		setup_clear_cpu_cap(X86_FEATURE_HLE);
> +		return TSX_CTRL_RTM_ALWAYS_ABORT;
> +	}

I don't really like hiding the setup_clear_cpu_cap() like this.  Right
now, all of the setup_clear_cpu_cap()'s are in a single function and
they are pretty easy to figure out.

This seems like logic that deserves to be appended down to the last if()
block of code in tsx_init() instead of squirreled away in a "get mode"
function.  Does this work?

        if (tsx_ctrl_state == TSX_CTRL_DISABLE) {
		...
        } else if (tsx_ctrl_state == TSX_CTRL_ENABLE) {
		...	
        } else if (tsx_ctrl_state == TSX_CTRL_RTM_ALWAYS_ABORT) {
		tsx_clear_cpuid();

		setup_clear_cpu_cap(X86_FEATURE_RTM);
		setup_clear_cpu_cap(X86_FEATURE_HLE);
	}