linux-kernel - Re: [PATCH v11 018/113] KVM: TDX: create/destroy VM structure

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Message-ID: <c7c73964f56a8dcc2ee7fff3ac128b9ef9f62e35.camel@intel.com>
Date:   Thu, 19 Jan 2023 20:40:40 +0000
From:   "Huang, Kai" <kai.huang@...el.com>
To:     "Christopherson,, Sean" <seanjc@...gle.com>
CC:     "dmatlack@...gle.com" <dmatlack@...gle.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Shahar, Sagi" <sagis@...gle.com>,
        "isaku.yamahata@...il.com" <isaku.yamahata@...il.com>,
        "Aktas, Erdem" <erdemaktas@...gle.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "Yamahata, Isaku" <isaku.yamahata@...el.com>,
        "sean.j.christopherson@...el.com" <sean.j.christopherson@...el.com>
Subject: Re: [PATCH v11 018/113] KVM: TDX: create/destroy VM structure

On Thu, 2023-01-19 at 15:29 +0000, Sean Christopherson wrote:
> On Thu, Jan 19, 2023, Huang, Kai wrote:
> > On Thu, 2023-01-12 at 08:31 -0800, isaku.yamahata@...el.com wrote:
> > > +static void tdx_clear_page(unsigned long page_pa)
> > > +{
> > > +	const void *zero_page = (const void *) __va(page_to_phys(ZERO_PAGE(0)));
> > > +	void *page = __va(page_pa);
> > > +	unsigned long i;
> > > +
> > > +	if (!static_cpu_has(X86_FEATURE_MOVDIR64B)) {
> > > +		clear_page(page);
> > > +		return;
> > > +	}
> > 
> > There might be below issues here:
> > 
> > 1) The kernel says static_cpu_has() should only be used in fast patch where each
> > cycle is counted, otherwise use boot_cpu_has().  I don't know whether here you
> > should use static_cpu_has().
> 
> That documentation is stale[*], go ahead and use cpu_feature_enabled().
> 
> https://lore.kernel.org/all/20221107211505.8572-1-bp@alien8.de

Thanks for the info :)

> 
> > 2) IIUC a CPU feature bit can be cleared by 'clearcpuid=xxx' kernel command
> 
> As you note below, using clearcpuid taints the kernel, i.e. any breakage due to
> clearcpuid is user error.

Agreed.