linux-kernel - Re: [PATCH 6/6] KVM: x86: Set KVM LASS based on hardware capability

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Date:   Tue, 25 Apr 2023 15:28:48 +0800
From:   Chao Gao <chao.gao@...el.com>
To:     Zeng Guang <guang.zeng@...el.com>
CC:     Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        H Peter Anvin <hpa@...or.com>, <kvm@...r.kernel.org>,
        <x86@...nel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 6/6] KVM: x86: Set KVM LASS based on hardware capability

On Thu, Apr 20, 2023 at 09:37:24PM +0800, Zeng Guang wrote:
>Host kernel may clear LASS capability in boot_cpu_data.x86_capability
>besides explicitly using clearcpuid parameter. That will cause guest
>not being able to manage LASS independently. So set KVM LASS directly
>based on hardware capability to eliminate the dependency.

...

>+	/* Set LASS based on hardware capability */
>+	if (cpuid_count_eax(7, 1) & F(LASS))
>+		kvm_cpu_cap_set(X86_FEATURE_LASS);
>+

What if LASS is cleared in boot_cpu_data because not all CPUs support LASS?

In arch/x86/kernel/cpu/common.c, identify_cpu() clears features which are
not supported by all CPUs:

	/*
	 * On SMP, boot_cpu_data holds the common feature set between
	 * all CPUs; so make sure that we indicate which features are
	 * common between the CPUs.  The first time this routine gets
	 * executed, c == &boot_cpu_data.
	 */
	if (c != &boot_cpu_data) {
		/* AND the already accumulated flags with these */
		for (i = 0; i < NCAPINTS; i++)
			boot_cpu_data.x86_capability[i] &= c->x86_capability[i];

LA57 seems to have the same issue. We may need to add some checks for LA57
in KVM's cpu hotplug callback.

> 	kvm_cpu_cap_init_kvm_defined(CPUID_7_1_EDX,
> 		F(AVX_VNNI_INT8) | F(AVX_NE_CONVERT) | F(PREFETCHITI)
> 	);
>-- 
>2.27.0
>