linux-kernel - Re: [PATCH v13 018/113] KVM: x86, tdx: Make KVM_CAP_MAX

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Message-ID: <20230329233258.GC1108448@ls.amr.corp.intel.com>
Date:   Wed, 29 Mar 2023 16:32:58 -0700
From:   Isaku Yamahata <isaku.yamahata@...il.com>
To:     Zhi Wang <zhi.wang.linux@...il.com>
Cc:     isaku.yamahata@...el.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, isaku.yamahata@...il.com,
        Paolo Bonzini <pbonzini@...hat.com>, erdemaktas@...gle.com,
        Sean Christopherson <seanjc@...gle.com>,
        Sagi Shahar <sagis@...gle.com>,
        David Matlack <dmatlack@...gle.com>,
        Kai Huang <kai.huang@...el.com>
Subject: Re: [PATCH v13 018/113] KVM: x86, tdx: Make KVM_CAP_MAX_VCPUS
 backend specific

On Sat, Mar 25, 2023 at 08:13:26PM +0200,
Zhi Wang <zhi.wang.linux@...il.com> wrote:

> On Sun, 12 Mar 2023 10:55:42 -0700
> isaku.yamahata@...el.com wrote:
> 
> > From: Isaku Yamahata <isaku.yamahata@...el.com>
> > 
> > TDX has its own limitation on the maximum number of vcpus that the guest
> > can accommodate.  Allow x86 kvm backend to implement its own KVM_ENABLE_CAP
> > handler and implement TDX backend for KVM_CAP_MAX_VCPUS.  user space VMM,
> > e.g. qemu, can specify its value instead of KVM_MAX_VCPUS.
> > 
> 
> I think enabling the cap here is actually "configuring the cap". KVM_CAP_MAX
> _VCPUS is actually always enabled whether userspace enables it or not. It
> would be nice to configure of the max VCPUS in kvm_arch_vm_ioctl() where
> routines of configuring a VM should belong. E.g. KVM_SET_MAX_VCPUS.

I'm not sure I understand your point.  Although KVM_ENABLE_CAP sounds like
only on/off feature, but it isn't. It's also used to set parameters. For
example, KVM_CAP_MAX_VCPU_ID.

KVM_SET_XXX is for run time feature. But the maxium number of vcpus is not
runtime changable. Set it at vm creation.
-- 
Isaku Yamahata <isaku.yamahata@...il.com>