linux-kernel - Re: [PATCH v3 04/21] x86/virt/tdx: Add skeleton for detecting and initializing TDX on demand

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Date:   Fri, 29 Apr 2022 11:44:19 +1200
From:   Kai Huang <kai.huang@...el.com>
To:     Dave Hansen <dave.hansen@...el.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     seanjc@...gle.com, pbonzini@...hat.com, len.brown@...el.com,
        tony.luck@...el.com, rafael.j.wysocki@...el.com,
        reinette.chatre@...el.com, dan.j.williams@...el.com,
        peterz@...radead.org, ak@...ux.intel.com,
        kirill.shutemov@...ux.intel.com,
        sathyanarayanan.kuppuswamy@...ux.intel.com,
        isaku.yamahata@...el.com
Subject: Re: [PATCH v3 04/21] x86/virt/tdx: Add skeleton for detecting and
 initializing TDX on demand

On Thu, 2022-04-28 at 07:27 -0700, Dave Hansen wrote:
> On 4/27/22 17:00, Kai Huang wrote:
> > On Wed, 2022-04-27 at 07:49 -0700, Dave Hansen wrote:
> > I think we can use pr_info_once() when all_cpus_booted() returns false, and get
> > rid of printing "SEAMRR not enabled" in seamrr_enabled().  How about below?
> > 
> > static bool seamrr_enabled(void)
> > {
> > 	if (!all_cpus_booted())
> > 		pr_info_once("Not all present CPUs have been booted.  Report
> > SEAMRR as not enabled");
> > 
> > 	return __seamrr_enabled();
> > }
> > 
> > And we don't print "SEAMRR not enabled".
> 
> That's better, but even better than that would be removing all that
> SEAMRR gunk in the first place.

Agreed.

> > > > > > +	/*
> > > > > > +	 * TDX requires at least two KeyIDs: one global KeyID to
> > > > > > +	 * protect the metadata of the TDX module and one or more
> > > > > > +	 * KeyIDs to run TD guests.
> > > > > > +	 */
> > > > > > +	return tdx_keyid_num >= 2;
> > > > > > +}
> > > > > > +
> > > > > > +static int __tdx_detect(void)
> > > > > > +{
> > > > > > +	/* The TDX module is not loaded if SEAMRR is disabled */
> > > > > > +	if (!seamrr_enabled()) {
> > > > > > +		pr_info("SEAMRR not enabled.\n");
> > > > > > +		goto no_tdx_module;
> > > > > > +	}
> > > > > 
> > > > > Why even bother with the SEAMRR stuff?  It sounded like you can "ping"
> > > > > the module with SEAMCALL.  Why not just use that directly?
> > > > 
> > > > SEAMCALL will cause #GP if SEAMRR is not enabled.  We should check whether
> > > > SEAMRR is enabled before making SEAMCALL.
> > > 
> > > So...  You could actually get rid of all this code.  if SEAMCALL #GP's,
> > > then you say, "Whoops, the firmware didn't load the TDX module
> > > correctly, sorry."
> > 
> > Yes we can just use the first SEAMCALL (TDH.SYS.INIT) to detect whether TDX
> > module is loaded.  If SEAMCALL is successful, the module is loaded.
> > 
> > One problem is currently the patch to flush cache for kexec() uses
> > seamrr_enabled() and tdx_keyid_sufficient() to determine whether we need to
> > flush the cache.  The reason is, similar to SME, the flush is done in
> > stop_this_cpu(), but the status of TDX module initialization is protected by
> > mutex, so we cannot use TDX module status in stop_this_cpu() to determine
> > whether to flush.
> > 
> > If that patch makes sense, I think we still need to detect SEAMRR?
> 
> Please go look at stop_this_cpu() closely.  What are the AMD folks doing
> for SME exactly?  Do they, for instance, do the WBINVD when the kernel
> used SME?  No, they just use a pretty low-level check if the processor
> supports SME.
> 
> Doing the same kind of thing for TDX is fine.  You could check the MTRR
> MSR bits that tell you if SEAMRR is supported and then read the MSR
> directly.  You could check the CPUID enumeration for MKTME or
> CPUID.B.0.EDX (I'm not even sure what this is but the SEAMCALL spec says
> it is part of SEAMCALL operation).

I am not sure about this CPUID either.  

> 
> Just like the SME test, it doesn't even need to be precise.  It just
> needs to be 100% accurate in that it is *ALWAYS* set for any system that
> might have dirtied cache aliases.
> 
> I'm not sure why you are so fixated on SEAMRR specifically for this.

I see.  I think I can simply use MTRR.SEAMRR bit check.  If CPU supports SEAMRR,
then basically it supports MKTME.

Is this look good for you?

	
> 
> 
> ...
> > "During initializing the TDX module, one step requires some SEAMCALL must be
> > done on all logical cpus enabled by BIOS, otherwise a later step will fail. 
> > Disable CPU hotplug during the initialization process to prevent any CPU going
> > offline during initializing the TDX module.  Note it is caller's responsibility
> > to guarantee all BIOS-enabled CPUs are in cpu_present_mask and all present CPUs
> > are online."
> 
> But, what if a CPU went offline just before this lock was taken?  What
> if the caller make sure all present CPUs are online, makes the call,
> then a CPU is taken offline.  The lock wouldn't do any good.
> 
> What purpose does the lock serve?

I thought cpus_read_lock() can prevent any CPU from going offline, no?


-- 
Thanks,
-Kai