linux-kernel - Re: [PATCH v11 021/113] KVM: TDX: Refuse to unplug the last cpu on the package

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Date:   Mon, 27 Feb 2023 13:48:09 -0800
From:   Isaku Yamahata <isaku.yamahata@...il.com>
To:     "Huang, Kai" <kai.huang@...el.com>
Cc:     "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Yamahata, Isaku" <isaku.yamahata@...el.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "Shahar, Sagi" <sagis@...gle.com>,
        "Aktas, Erdem" <erdemaktas@...gle.com>,
        "isaku.yamahata@...il.com" <isaku.yamahata@...il.com>,
        "dmatlack@...gle.com" <dmatlack@...gle.com>,
        "Christopherson,, Sean" <seanjc@...gle.com>
Subject: Re: [PATCH v11 021/113] KVM: TDX: Refuse to unplug the last cpu on
 the package

On Mon, Jan 16, 2023 at 10:23:16AM +0000,
"Huang, Kai" <kai.huang@...el.com> wrote:

> On Thu, 2023-01-12 at 08:31 -0800, isaku.yamahata@...el.com wrote:
> > From: Isaku Yamahata <isaku.yamahata@...el.com>
> > 
> > In order to reclaim TDX HKID, (i.e. when deleting guest TD), needs to call
> > TDH.PHYMEM.PAGE.WBINVD on all packages.  If we have used TDX HKID, refuse
> > to offline the last online cpu. Add arch callback for cpu offline.
> 
> I think it is worth to talk about suspend staff, i.e. why we only refuse to
> offline the last cpu when there's active TD, but not choose to offline the last
> cpu when TDX is enabled in KVM.  People may not be able to understand
> immediately the reason behind this design.

Updated the comment.

> Btw, I certainly don't want to speak for Sean, but it seems this was suggested
> by Sean?  If so, add a 'Suggested-by' tag?

Added suggested-by.

> > 
> > Signed-off-by: Isaku Yamahata <isaku.yamahata@...el.com>
> > ---
> > 
> 
> [snip]
> 
> > +
> > +int tdx_offline_cpu(void)
> > +{
> > +	int curr_cpu = smp_processor_id();
> > +	cpumask_var_t packages;
> > +	int ret = 0;
> > +	int i;
> > +
> > +	if (!atomic_read(&nr_configured_hkid))
> > +		return 0;
> 
> As mentioned above, I think it also worth to add some comment here.  When people
> are trying to understand some code, I think mostly they are just going to look
> at the code itself, but won't use 'git blame' to dig out the entire changelog to
> understand some code.

Makes sense. Added a comment.
-- 
Isaku Yamahata <isaku.yamahata@...il.com>