linux-kernel - Re: [PATCH RESEND] KVM: x86/pmu: Make top-down.slots event unavailable in supported leaf

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Message-ID: <b57f1785-e61f-20dd-49e1-76dfc1917df8@redhat.com>
Date:   Fri, 7 Jan 2022 17:59:49 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Like Xu <like.xu.linux@...il.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH RESEND] KVM: x86/pmu: Make top-down.slots event
 unavailable in supported leaf

On 1/5/22 06:07, Like Xu wrote:
> +		/*
> +		 * The 8th Intel pre-defined architectural event (Topdown Slots) will be supported
> +		 * if the 4th fixed counter exists && EAX[31:24] > 7 && EBX[7] = 0.
> +		 *
> +		 * Currently, KVM needs to set EAX[31:24] < 8 or EBX[7] == 1
> +		 * to make this event unavailable in a consistent way.
> +		 */
> +		if (edx.split.num_counters_fixed < 4) {
> +			if (eax.split.mask_length > 7)
> +				eax.split.mask_length--;
> +			if (eax.split.mask_length > 7)
> +				cap.events_mask |= BIT_ULL(7);
> +		}
> +

The first "> 7" is wrong; it should be == 8, shouldn't it?  Something like

if (edx.split.num_counters_fixed < 4 && eax.split.mask_length >= 8) {
	if (eax.split.mask_length == 8)
		eax.split.mask_length--;
	else
		cap.events_mask |= BIT_ULL(7);
}

is what you mean, I think?

Paolo