Messages in this thread | | | From | Juergen Gross <> | Subject | Re: [PATCH v3 4/4] x86/kvm: add boot parameter for setting max number of vcpus per guest | Date | Thu, 18 Nov 2021 16:15:34 +0100 |
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On 18.11.21 16:05, Sean Christopherson wrote: > On Thu, Nov 18, 2021, Juergen Gross wrote: >> On 17.11.21 21:57, Sean Christopherson wrote: >>> Rather than makes this a module param, I would prefer to start with the below >>> patch (originally from TDX pre-enabling) and then wire up a way for userspace to >>> _lower_ the max on a per-VM basis, e.g. add a capability. >> >> The main reason for this whole series is a request by a partner >> to enable huge VMs on huge machines (huge meaning thousands of >> vcpus on thousands of physical cpus). >> >> Making this large number a compile time setting would hurt all >> the users who have more standard requirements by allocating the >> needed resources even on small systems, so I've switched to a boot >> parameter in order to enable those huge numbers only when required. >> >> With Marc's series to use an xarray for the vcpu pointers only the >> bitmaps for sending IRQs to vcpus are left which need to be sized >> according to the max vcpu limit. Your patch below seems to be fine, but >> doesn't help for that case. > > Ah, you want to let userspace define a MAX_VCPUS that goes well beyond the current > limit without negatively impacting existing setups. My idea of a per-VM capability
Correct.
> still works, it would simply require separating the default max from the absolute > max, which this patch mostly does already, it just neglects to set an absolute max. > > Which is a good segue into pointing out that if a module param is added, it needs > to be sanity checked against a KVM-defined max. The admin may be trusted to some > extent, but there is zero reason to let userspace set max_vcspus to 4 billion. > At that point, it really is just a param vs. capability question.
I agree. Capping it at e.g. 65536 would probably be a good idea.
> I like the idea of a capability because there are already two known use cases, > arm64's GIC and x86's TDX, and it could also be used to reduce the kernel's footprint > for use cases that run large numbers of smaller VMs. > > The other alternative would be to turn KVM_MAX_VCPUS into a Kconfig knob. I assume
I like combining the capping and a Kconfig knob. So let the distro (or whoever is building the kernel) decide, which is the max allowed value (e.g. above 65536 per default).
> the partner isn't running a vanilla distro build and could set it as they see fit.
And here you are wrong. They'd like to use standard SUSE Linux (SLE).
Juergen [unhandled content-type:application/pgp-keys][unhandled content-type:application/pgp-signature] | |