linux-kernel - Re: [PATCH v19 037/130] KVM: TDX: Make KVM_CAP_MAX

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Date: Wed, 19 Jun 2024 04:52:45 +0000
From: "Huang, Kai" <kai.huang@...el.com>
To: "seanjc@...gle.com" <seanjc@...gle.com>
CC: "Zhang, Tina" <tina.zhang@...el.com>, "Yuan, Hang" <hang.yuan@...el.com>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>, "Chen, Bo2" <chen.bo@...el.com>,
	"sagis@...gle.com" <sagis@...gle.com>, "isaku.yamahata@...ux.intel.com"
	<isaku.yamahata@...ux.intel.com>, "Aktas, Erdem" <erdemaktas@...gle.com>,
	"isaku.yamahata@...il.com" <isaku.yamahata@...il.com>, "pbonzini@...hat.com"
	<pbonzini@...hat.com>, "Yamahata, Isaku" <isaku.yamahata@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v19 037/130] KVM: TDX: Make KVM_CAP_MAX_VCPUS backend
 specific

On Tue, 2024-06-18 at 22:19 +0000, Huang, Kai wrote:
> On Tue, 2024-06-18 at 07:49 -0700, Sean Christopherson wrote:
> > On Tue, Jun 18, 2024, Kai Huang wrote:
> > > On 15/06/2024 12:04 pm, Sean Christopherson wrote:
> > > > On Fri, Jun 14, 2024, Kai Huang wrote:
> > > > > > - The "max_vcpus_per_td" can be different depending on module versions. In
> > > > > > practice it reflects the maximum physical logical cpus that all the
> > > > > > platforms (that the module supports) can possibly have.
> > > > 
> > > > It's a reasonable restriction, e.g. KVM_CAP_NR_VCPUS is already capped at number
> > > > of online CPUs, although userspace is obviously allowed to create oversubscribed
> > > > VMs.
> > > > 
> > > > I think the sane thing to do is document that TDX VMs are restricted to the number
> > > > of logical CPUs in the system, have KVM_CAP_MAX_VCPUS enumerate exactly that, and
> > > > then sanity check that max_vcpus_per_td is greater than or equal to what KVM
> > > > reports for KVM_CAP_MAX_VCPUS. >
> > > > Stating that the maximum number of vCPUs depends on the whims TDX module doesn't
> > > > provide a predictable ABI for KVM, i.e. I don't want to simply forward TDX's
> > > > max_vcpus_per_td to userspace.
> > > 
> > > This sounds good to me.  I think it should be also OK for client too, if TDX
> > > ever gets supported for client.
> > > 
> > > IIUC we can consult the @nr_cpu_ids or num_possible_cpus() to get the
> > > "number of logical CPUs in the system".  And we can reject to use the TDX
> > > module if 'max_vcpus_per_td' turns to be smaller.
> > 
> > I assume TDX is incompatible with actual physical CPU hotplug?  
> > 
> 
> Correct.
> 
> > If so, we can and
> > should use num_present_cpus().  
> > 
> 
> On TDX platform num_present_cpus() and num_possible_cpus() should be just
> identical, because TDX requires BIOS to mark all all physical LPs the
> platform as enabled, and TDX doesn't support physical CPU hotplug.
> 
> Using num_present_cpus() w/o holding CPU hotplug lock is a little bit
> annoying from code's perspective, but it's OK to me.  We can add a comment
> saying TDX doesn't support physical CPU hotplug.
> 
> > If  loading the TDX module completely disables
> > onlining CPUs, then we can use num_online_cpus().

Missed this one.

No loading/initializing TDX module doesn't disable onlining CPUs.  TDX
module can be initialized on a subset of physical logical CPUs.  Also,
after module initialization, logical CPU can be put to offline and then
brought back to online again.