linux-kernel - Re: [PATCH v3 1/4] x86/kvm: add boot parameter for adding vcpu-id bits

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

Date:   Wed, 17 Nov 2021 07:59:29 +0100
From:   Juergen Gross <jgross@...e.com>
To:     kvm@...r.kernel.org, x86@...nel.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org
Cc:     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>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH v3 1/4] x86/kvm: add boot parameter for adding vcpu-id
 bits

On 16.11.21 15:10, 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_IDS) 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 2 in order to support
> today's possible topologies. The special value of -1 will use the
> number of bits needed for a guest with the current host's topology.
> 
> Calculating the maximum vcpu-id dynamically requires to allocate the
> arrays using KVM_MAX_VCPU_IDS as the size dynamically.
> 
> Signed-of-by: Juergen Gross <jgross@...e.com>

Just thought about vcpu-ids a little bit more.

It would be possible to replace the topology games completely by an
arbitrary rather high vcpu-id limit (65536?) and to allocate the memory
depending on the max vcpu-id just as needed.

Right now the only vcpu-id dependent memory is for the ioapic consisting
of a vcpu-id indexed bitmap and a vcpu-id indexed byte array (vectors).

We could start with a minimal size when setting up an ioapic and extend
the areas in case a new vcpu created would introduce a vcpu-id outside
the currently allocated memory. Both arrays are protected by the ioapic
specific lock (at least I couldn't spot any unprotected usage when
looking briefly into the code), so reallocating those arrays shouldn't
be hard. In case of ENOMEM the related vcpu creation would just fail.

Thoughts?


Juergen

Download attachment "OpenPGP_0xB0DE9DD628BF132F.asc" of type "application/pgp-keys" (3092 bytes)

Download attachment "OpenPGP_signature" of type "application/pgp-signature" (496 bytes)