linux-kernel - Re: [PATCH v3 1/2] x86/cpu/intel: Fix MTRR verification for TME enabled platforms

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Message-ID: <87a5sizgr8.fsf@jcompost-mobl.amr.corp.intel.com>
Date:   Mon, 16 Oct 2023 09:14:35 -0700
From:   "Compostella, Jeremy" <jeremy.compostella@...el.com>
To:     <kirill.shutemov@...ux.intel.com>
Cc:     "Huang, Kai" <kai.huang@...el.com>,
        "mingo@...nel.org" <mingo@...nel.org>,
        "Li, Xin3" <xin3.li@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>, "bp@...en8.de" <bp@...en8.de>
Subject: Re: [PATCH v3 1/2] x86/cpu/intel: Fix MTRR verification for TME
 enabled platforms

<kirill.shutemov@...ux.intel.com> writes:

> On Fri, Oct 13, 2023 at 04:03:02PM -0700, Compostella, Jeremy wrote:
>> "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com> writes:
>> > On Tue, Oct 03, 2023 at 02:06:52AM +0000, Huang, Kai wrote:
>> >> On Tue, 2023-10-03 at 01:47 +0300, kirill.shutemov@...ux.intel.com wrote:
>> >> > On Fri, Sep 29, 2023 at 09:14:00AM +0000, Huang, Kai wrote:
>> >> > > On Thu, 2023-09-28 at 15:30 -0700, Compostella, Jeremy wrote:
>> >> > > > On TME enabled platform, BIOS publishes MTRR taking into account Total
>> >> > > > Memory Encryption (TME) reserved bits.
>> >> > > > 
>> >> > > > generic_get_mtrr() performs a sanity check of the MTRRs relying on the
>> >> > > > `phys_hi_rsvd' variable which is set using the cpuinfo_x86 structure
>> >> > > > `x86_phys_bits' field.  But at the time the generic_get_mtrr()
>> >> > > > function is ran the `x86_phys_bits' has not been updated by
>> >> > > > detect_tme() when TME is enabled.
>> >> > > > 
>> >> > > > Since the x86_phys_bits does not reflect yet the real maximal physical
>> >> > > > address size yet generic_get_mtrr() complains by logging the following
>> >> > > > messages.
>> >> > > > 
>> >> > > >     mtrr: your BIOS has configured an incorrect mask, fixing it.
>> >> > > >     mtrr: your BIOS has configured an incorrect mask, fixing it.
>> >> > > >     [...]
>> >> > > > 
>> >> > > > In such a situation, generic_get_mtrr() returns an incorrect size but
>> >> > > > no side effect were observed during our testing.
>> >> > > > 
>> >> > > > For `x86_phys_bits' to be updated before generic_get_mtrr() runs,
>> >> > > > move the detect_tme() call from init_intel() to early_init_intel().
>> >> > > 
>> >> > > Hi,
>> >> > > 
>> >> > > This move looks good to me, but +Kirill who is the author of detect_tme() for
>> >> > > further comments.
>> >> > > 
>> >> > > Also I am not sure whether it's worth to consider to move this to
>> >> > > get_cpu_address_sizes(), which calculates the virtual/physical address sizes. 
>> >> > > Thus it seems anything that can impact physical address size
>> >> > > could be put there.
>> >> > 
>> >> > Actually, I am not sure how this patch works. AFAICS after the patch we
>> >> > have the following callchain:
>> >> > 
>> >> > early_identify_cpu()
>> >> >   this_cpu->c_early_init() (which is early_init_init())
>> >> >     detect_tme()
>> >> >       c->x86_phys_bits -= keyid_bits;
>> >> >   get_cpu_address_sizes(c);
>> >> >     c->x86_phys_bits = eax & 0xff;
>> >> > 
>> >> > Looks like get_cpu_address_sizes() would override what detect_tme() does.
>> >> 
>> >> After this patch, early_identify_cpu() calls get_cpu_address_sizes() first and
>> >> then calls c_early_init(), which calls detect_tme().
>> >> 
>> >> So looks no override.  No?
>> 
>> No override indeed as get_cpu_address_sizes() is always called before
>> early_init_intel or init_intel().
>> 
>> - init/main.c::start_kernel()
>>   - arch/x86/kernel/setup.c::setup_arch()
>>     - arch/x86/kernel/cpu/common.c::early_cpu_init()
>>       - early_identify_cpu()
>>         - get_cpu_address_sizes(c)
>>           c->x86_phys_bits = eax & 0xff;
>>         - arch/x86/kernel/cpu/intel.c::early_init_intel()
>>           - detect_tme()
>>             c->x86_phys_bits -= keyid_bits;
>
> Hmm.. Do I read it wrong:
>
> 	static void __init early_identify_cpu(struct cpuinfo_x86 *c)
> 	{
> 	...
> 		/* cyrix could have cpuid enabled via c_identify()*/
> 		if (have_cpuid_p()) {
> 		...
> 		        // Here we call early_intel_init()
> 			if (this_cpu->c_early_init)
> 				this_cpu->c_early_init(c);
> 			...
> 		}
>
> 		get_cpu_address_sizes(c);
> 	...
> 	}
>
> ?
>
> As far as I see get_cpu_address_sizes() called after early_intel_init().

On `58720809f527 v6.6-rc6 6.6-rc6 2de3c93ef41b' is what I have:

,----
| 1599  /* cyrix could have cpuid enabled via c_identify()*/
| 1600  if (have_cpuid_p()) {
| 1601  	cpu_detect(c);
| 1602  	get_cpu_vendor(c);
| 1603  	get_cpu_cap(c);
| 1604  	get_cpu_address_sizes(c);                   <= called first
| 1605  	setup_force_cpu_cap(X86_FEATURE_CPUID);
| 1606  	cpu_parse_early_param();
| 1607  
| 1608  	if (this_cpu->c_early_init)
| 1609  		this_cpu->c_early_init(c);
| 1610  
| 1611  	c->cpu_index = 0;
| 1612  	filter_cpuid_features(c, false);
| 1613  
| 1614  	if (this_cpu->c_bsp_init)
| 1615  		this_cpu->c_bsp_init(c);
| 1616  } else {
| 1617  	setup_clear_cpu_cap(X86_FEATURE_CPUID);
| 1618  }
`----
Listing 1: arch/x86/kernel/cpu/common.c

=> get_cpu_address_sizes() is called first which is also conform to my
   experiments and instrumentation.

-- 
Jeremy
One Emacs to rule them all