linux-kernel - Re: [PATCH v2 2/6] x86/kvm: add boot parameter for adding vcpu-id bits

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Date:   Fri, 3 Sep 2021 15:48:24 -0400
From:   Eduardo Habkost <ehabkost@...hat.com>
To:     Juergen Gross <jgross@...e.com>
Cc:     kvm@...r.kernel.org, x86@...nel.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, maz@...nel.org,
        Jonathan Corbet <corbet@....net>,
        Paolo Bonzini <pbonzini@...hat.com>,
        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>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH v2 2/6] x86/kvm: add boot parameter for adding vcpu-id
 bits

On Fri, Sep 03, 2021 at 03:08:03PM +0200, Juergen Gross wrote:
> Today the maximum vcpu-id of a kvm guest's vcpu on x86 systems is set
> via a #define in a header file.
> 
> In order to support higher vcpu-ids without generally increasing the
> memory consumption of guests on the host (some guest structures contain
> arrays sized by KVM_MAX_VCPU_ID) add a boot parameter for adding some
> bits to the vcpu-id. Additional bits are needed as the vcpu-id is
> constructed via bit-wise concatenation of socket-id, core-id, etc.
> As those ids maximum values are not always a power of 2, the vcpu-ids
> are sparse.
> 
> The additional number of bits needed is basically the number of
> topology levels with a non-power-of-2 maximum value, excluding the top
> most level.
> 
> The default value of the new parameter will be to take the correct
> setting from the host's topology.

Having the default depend on the host topology makes the host
behaviour unpredictable (which might be a problem when migrating
VMs from another host with a different topology).  Can't we just
default to 2?

> 
> Calculating the maximum vcpu-id dynamically requires to allocate the
> arrays using KVM_MAX_VCPU_ID as the size dynamically.
> 
> Signed-of-by: Juergen Gross <jgross@...e.com>
> ---
> V2:
> - switch to specifying additional bits (based on comment by Vitaly
>   Kuznetsov)
> 
> Signed-off-by: Juergen Gross <jgross@...e.com>
> ---
[...]
>  #define KVM_MAX_VCPUS 288
>  #define KVM_SOFT_MAX_VCPUS 240
> -#define KVM_MAX_VCPU_ID 1023
> +#define KVM_MAX_VCPU_ID kvm_max_vcpu_id()
[...]
> +unsigned int kvm_max_vcpu_id(void)
> +{
> +	int n_bits = fls(KVM_MAX_VCPUS - 1);
> +
> +	if (vcpu_id_add_bits < -1 || vcpu_id_add_bits > (32 - n_bits)) {
> +		pr_err("Invalid value of vcpu_id_add_bits=%d parameter!\n",
> +		       vcpu_id_add_bits);
> +		vcpu_id_add_bits = -1;
> +	}
> +
> +	if (vcpu_id_add_bits >= 0) {
> +		n_bits += vcpu_id_add_bits;
> +	} else {
> +		n_bits++;		/* One additional bit for core level. */
> +		if (topology_max_die_per_package() > 1)
> +			n_bits++;	/* One additional bit for die level. */
> +	}
> +
> +	if (!n_bits)
> +		n_bits = 1;
> +
> +	return (1U << n_bits) - 1;

The largest possible VCPU ID is not KVM_MAX_VCPU_ID,
it's (KVM_MAX_VCPU_ID - 1).  This is enforced by
kvm_vm_ioctl_create_vcpu().

That would mean KVM_MAX_VCPU_ID should be (1 << n_bits) instead
of ((1 << n_bits) - 1), wouldn't it?


> +}
> +EXPORT_SYMBOL_GPL(kvm_max_vcpu_id);
> +
>  /*
>   * Restoring the host value for MSRs that are only consumed when running in
>   * usermode, e.g. SYSCALL MSRs and TSC_AUX, can be deferred until the CPU
> -- 
> 2.26.2
> 

-- 
Eduardo