linux-kernel - Re: [PATCH V2] KVM/x86: Increase max vcpu number to 8192

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Message-ID: <20170821155200.GA17079@flask>
Date:   Mon, 21 Aug 2017 17:52:00 +0200
From:   Radim Krčmář <rkrcmar@...hat.com>
To:     Lan Tianyu <tianyu.lan@...el.com>
Cc:     pbonzini@...hat.com, tglx@...utronix.de, mingo@...hat.com,
        hpa@...or.com, x86@...nel.org, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH V2] KVM/x86: Increase max vcpu number to 8192

2017-08-15 20:43-0400, Lan Tianyu:
> For HPC usage case, it will create a huge VM with vcpus number as same as host
> cpus and this requires more vcpus support in a single VM. This patch is to
> increase max vcpu number from 288 to 8192 which is current default maximum cpu
> number for Linux kernel.
> 
> Signed-off-by: Lan Tianyu <tianyu.lan@...el.com>
> ---
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> @@ -35,7 +35,7 @@
>  #include <asm/asm.h>
>  #include <asm/kvm_page_track.h>
>  
> -#define KVM_MAX_VCPUS 288
> +#define KVM_MAX_VCPUS 8192
>  #define KVM_SOFT_MAX_VCPUS 240
>  #define KVM_MAX_VCPU_ID 1023

We will also need to raise KVM_MAX_VCPU_ID to actually allow that many
VCPUs.  If I count correctly, it is impossible to have ID bigger than
this:

 #define KVM_MAX_VCPU_ID (KVM_MAX_VCPUS * 4)

(And we can also use NR_CPUS instead of 8192.)

Thanks.