linux-kernel - Re: kconfig does not accept a lower value for KVM_MAX_NR

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [day] [month] [year] [list]

Message-ID: <Z4_DYfJKl3jqzU_Y@google.com>
Date: Tue, 21 Jan 2025 07:55:13 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Bagas Sanjaya <bagasdotme@...il.com>
Cc: "Toralf Förster" <toralf.foerster@....de>, 
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, Linux KVM <kvm@...r.kernel.org>, 
	Paolo Bonzini <pbonzini@...hat.com>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, 
	Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, 
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: kconfig does not accept a lower value for KVM_MAX_NR_VCPUS

On Mon, Jan 20, 2025, Bagas Sanjaya wrote:
> On Sun, Jan 19, 2025 at 12:04:04PM +0100, Toralf Förster wrote:
> > I was wondering why I cannot put a lower value here during make oldconfig:

The lower limit of 1024 in the Kconfig exists to ensure backwards compatibility.
CONFIG_KVM_MAX_NR_VCPUS is effectively exposed to userspace via KVM_CAP_MAX_VCPUS,
and prior to the Kconfig KVM hardcoded KVM_CAP_MAX_VCPUS to 1024.

I did float the idea of setting the range to 1 - 4096, but we opted to go with
the conservative approach because the benefits are relatively minor, and we didn't
want to risk indirectly breaking userspace.

> >   Maximum number of vCPUs per KVM guest (KVM_MAX_NR_VCPUS) [1024] (NEW) 16
> >   Maximum number of vCPUs per KVM guest (KVM_MAX_NR_VCPUS) [1024] (NEW) 8
> >   Maximum number of vCPUs per KVM guest (KVM_MAX_NR_VCPUS) [1024] (NEW)
> > 
> 
> Hi Toralf,
> 
> From arch/x86/kvm/Kconfig:
> 
> >config KVM_MAX_NR_VCPUS
> >	int "Maximum number of vCPUs per KVM guest"
> >	depends on KVM
> >	range 1024 4096
> >	default 4096 if MAXSMP
> >	default 1024
> >	help
> >	  Set the maximum number of vCPUs per KVM guest. Larger values will increase
> >	  the memory footprint of each KVM guest, regardless of how many vCPUs are
> >	  created for a given VM.
> 
> I don't know your use case, but you can safely choose the default (1024).
> 
> Thanks.
> 
> -- 
> An old man doll... just what I always wanted! - Clara