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Message-ID: <Y8lh59ZYZkq4fsOX@google.com>
Date: Thu, 19 Jan 2023 15:29:43 +0000
From: Sean Christopherson <seanjc@...gle.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>,
"sean.j.christopherson@...el.com" <sean.j.christopherson@...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>
Subject: Re: [PATCH v11 018/113] KVM: TDX: create/destroy VM structure
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
> 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.
> line, so looks you should use CPUID directly otherwise the MOVDIR64B below can
> be unintentionally skipped. In practice w/o doing MOVDIR64B is fine since KeyID
> 0 doesn't have integrity enabled, but for the purpose you want to achieve
> checking real CPUID should be better.
>
> But maybe you don't want to do CPUID check here each time when reclaiming a
> page. In that case you can do CPUID during module initialization and cache
> whether MOVDIR64B is truly present. static_key is a good fit for this purpose
> too I think.
>
> But I am also seeing below in the kernel documentation:
>
> clearcpuid=X[,X...] [X86]
> ......
> Note that using this option will taint your kernel.
> Also note that user programs calling CPUID directly
> or using the feature without checking anything
> will still see it. This just prevents it from
> being used by the kernel or shown in /proc/cpuinfo.
> Also note the kernel might malfunction if you disable
> some critical bits.
>
> So the kernel is claiming using this will taint the kernel and it can even
> malfunction. So maybe it's OK to use static_cpu_has()/boot_cpu_has().
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