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

From: Sean Christopherson <seanjc@google.com>
To: Juergen Gross <jgross@suse.com>
Cc: kvm@vger.kernel.org, x86@kernel.org, linux-doc@vger.kernel.org,
	linux-kernel@vger.kernel.org, Jonathan Corbet <corbet@lwn.net>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	Wanpeng Li <wanpengli@tencent.com>,
	Jim Mattson <jmattson@google.com>, Joerg Roedel <joro@8bytes.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	"H. Peter Anvin" <hpa@zytor.com>
Subject: Re: [PATCH v3 1/4] x86/kvm: add boot parameter for adding vcpu-id bits
Date: Wed, 17 Nov 2021 23:46:31 +0000	[thread overview]
Message-ID: <YZWUV2jvoOS9RSq8@google.com> (raw)
In-Reply-To: <7f10b8b4-e753-c977-f201-5ef17a6e81c8@suse.com>

On Wed, Nov 17, 2021, Juergen Gross wrote:
> 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@suse.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?

Why not have userspace state the max vcpu_id it intends to creates on a per-VM
basis?  Same end result, but doesn't require the complexity of reallocating the
I/O APIC stuff.