linux-kernel - Re: [PATCH 2/2] KVM: SVM: Extend host physical APIC ID field to support more than 8-bit

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Date:   Wed, 17 Nov 2021 14:06:59 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Suravee Suthikulpanit <suravee.suthikulpanit@....com>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org, x86@...nel.org
Cc:     joro@...tes.org, tglx@...utronix.de, mingo@...hat.com,
        bp@...en8.de, peterz@...radead.org, hpa@...or.com,
        thomas.lendacky@....com, jon.grimm@....com
Subject: Re: [PATCH 2/2] KVM: SVM: Extend host physical APIC ID field to
 support more than 8-bit

On 11/10/21 11:18, Suravee Suthikulpanit wrote:
> +	if (level_type != 2 || !x2apic_mode) {
> +		avic_host_physical_id_mask = 0xffULL;
> +		goto out;
> +	}
> +
> +	core_mask_width = eax & 0xF;
> +
> +	max_phys_mask_width = get_count_order(apic_get_max_phys_apicid());
> +
> +	/*
> +	 * Sanity check to ensure core_mask_width for a processor does not
> +	 * exceed the calculated mask.
> +	 */
> +	if (WARN_ON(core_mask_width > max_phys_mask_width))
> +		return -EINVAL;

Can it just use apic_get_max_phys_apicid() in x2apic mode, and 0xff in 
!x2apic mode?  I'm not sure why you need to check CPUID[0xb,0x1].

Paolo