Date:   Fri, 2 Dec 2022 15:56:49 +0100
From:   Juergen Gross <jgross@...e.com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v5 13/16] x86: decouple PAT and MTRR handling

On 02.12.22 15:33, Kirill A. Shutemov wrote:
> On Fri, Dec 02, 2022 at 02:39:58PM +0100, Juergen Gross wrote:
>> On 02.12.22 14:27, Kirill A. Shutemov wrote:
>>> On Fri, Dec 02, 2022 at 06:56:47AM +0100, Juergen Gross wrote:
>>>> On 02.12.22 00:57, Kirill A. Shutemov wrote:
>>>>> On Thu, Dec 01, 2022 at 05:33:28PM +0100, Juergen Gross wrote:
>>>>>> On 01.12.22 17:26, Kirill A. Shutemov wrote:
>>>>>>> On Wed, Nov 02, 2022 at 08:47:10AM +0100, Juergen Gross wrote:
>>>>>>>> Today PAT is usable only with MTRR being active, with some nasty tweaks
>>>>>>>> to make PAT usable when running as Xen PV guest, which doesn't support
>>>>>>>> MTRR.
>>>>>>>>
>>>>>>>> The reason for this coupling is, that both, PAT MSR changes and MTRR
>>>>>>>> changes, require a similar sequence and so full PAT support was added
>>>>>>>> using the already available MTRR handling.
>>>>>>>>
>>>>>>>> Xen PV PAT handling can work without MTRR, as it just needs to consume
>>>>>>>> the PAT MSR setting done by the hypervisor without the ability and need
>>>>>>>> to change it. This in turn has resulted in a convoluted initialization
>>>>>>>> sequence and wrong decisions regarding cache mode availability due to
>>>>>>>> misguiding PAT availability flags.
>>>>>>>>
>>>>>>>> Fix all of that by allowing to use PAT without MTRR and by reworking
>>>>>>>> the current PAT initialization sequence to match better with the newly
>>>>>>>> introduced generic cache initialization.
>>>>>>>>
>>>>>>>> This removes the need of the recently added pat_force_disabled flag, so
>>>>>>>> remove the remnants of the patch adding it.
>>>>>>>>
>>>>>>>> Signed-off-by: Juergen Gross <jgross@...e.com>
>>>>>>>
>>>>>>> This patch breaks boot for TDX guest.
>>>>>>>
>>>>>>> Kernel now tries to set CR0.CD which is forbidden in TDX guest[1] and
>>>>>>> causes #VE:
>>>>>>>
>>>>>>> 	tdx: Unexpected #VE: 28
>>>>>>> 	VE fault: 0000 [#1] PREEMPT SMP NOPTI
>>>>>>> 	CPU: 0 PID: 0 Comm: swapper Not tainted 6.1.0-rc1-00015-gadfe7512e1d0 #2646
>>>>>>> 	Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015
>>>>>>> 	RIP: 0010:native_write_cr0 (arch/x86/kernel/cpu/common.c:427)
>>>>>>> 	 Call Trace:
>>>>>>> 	  <TASK>
>>>>>>> 	 ? cache_disable (arch/x86/include/asm/cpufeature.h:173 arch/x86/kernel/cpu/cacheinfo.c:1085)
>>>>>>> 	 ? cache_cpu_init (arch/x86/kernel/cpu/cacheinfo.c:1132 (discriminator 3))
>>>>>>> 	 ? setup_arch (arch/x86/kernel/setup.c:1079)
>>>>>>> 	 ? start_kernel (init/main.c:279 (discriminator 3) init/main.c:477 (discriminator 3) init/main.c:960 (discriminator 3))
>>>>>>> 	 ? load_ucode_bsp (arch/x86/kernel/cpu/microcode/core.c:155)
>>>>>>> 	 ? secondary_startup_64_no_verify (arch/x86/kernel/head_64.S:358)
>>>>>>> 	  </TASK>
>>>>>>>
>>>>>>> Any suggestion how to fix it?
>>>>>>>
>>>>>>> [1] Section 10.6.1. "CR0", https://cdrdv2.intel.com/v1/dl/getContent/733568
>>>>>>
>>>>>> What was the solution before?
>>>>>>
>>>>>> I guess MTRR was disabled, so there was no PAT, too?
>>>>>
>>>>> Right:
>>>>>
>>>>> Linus' tree:
>>>>>
>>>>> [    0.002589] last_pfn = 0x480000 max_arch_pfn = 0x10000000000
>>>>> [    0.003976] Disabled
>>>>> [    0.004452] x86/PAT: MTRRs disabled, skipping PAT initialization too.
>>>>> [    0.005856] CPU MTRRs all blank - virtualized system.
>>>>> [    0.006915] x86/PAT: Configuration [0-7]: WB  WT  UC- UC  WB  WT  UC- UC
>>>>>
>>>>> tip/master:
>>>>>
>>>>> [    0.003443] last_pfn = 0x20b8e max_arch_pfn = 0x10000000000
>>>>> [    0.005220] Disabled
>>>>> [    0.005818] x86/PAT: Configuration [0-7]: WB  WC  UC- UC  WB  WP  UC- WT
>>>>> [    0.007752] tdx: Unexpected #VE: 28
>>>>>
>>>>> The dangling "Disabled" comes mtrr_bp_init().
>>>>>
>>>>>
>>>>>> If this is the case, you can go the same route as Xen PV guests do.
>>>>>
>>>>> Any reason X86_FEATURE_HYPERVISOR cannot be used instead of
>>>>> X86_FEATURE_XENPV there?
>>>>>
>>>>> Do we have any virtualized platform that supports it?
>>>>
>>>> Yes, of course. Any hardware virtualized guest should be able to use it,
>>>> obviously TDX guests are the first ones not being able to do so.
>>>>
>>>> And above dmesg snipplets are showing rather nicely that not disabling
>>>> PAT completely should be a benefit for TDX guests, as all caching modes
>>>> would be usable (the PAT MSR seems to be initialized quite fine).
>>>>
>>>> Instead of X86_FEATURE_XENPV we could introduce something like
>>>> X86_FEATURE_PAT_READONLY, which could be set for Xen PV guests and for
>>>> TDX guests.
>>>
>>> Technically, the MSR is writable on TDX. But it seems there's no way to
>>> properly change it, following the protocol of changing on MP systems.
>>
>> Why not? I don't see why it is possible in a non-TDX system, but not within
>> a TDX guest.
> 
> Because the protocol you described below requires setting CR0.CD which is
> not allowed in TDX guest and causes #VE.

Hmm, yes, seems to be a valid reason. :-)

> 
>>> Although, I don't quite follow what role cache disabling playing on system
>>> with self-snoop support. Hm?
>>
>> It is the recommended way to do it. See SDM Vol. 3 Chapter 11 ("Memory Cache
>> Control"):
>>
>> The operating system is responsible for insuring that changes to a PAT entry
>> occur in a manner that maintains the consistency of the processor caches and
>> translation lookaside buffers (TLB). This is accomplished by following the
>> procedure as specified in Section 11.11.8, “MTRR Considerations in MP Systems,
>> ”for changing the value of an MTRR in a multiple processor system. It requires
>> a specific sequence of operations that includes flushing the processors caches
>> and TLBs.
>>
>> And the sequence for the MTRRs is:
>>
>> 1. Broadcast to all processors to execute the following code sequence.
>> 2. Disable interrupts.
>> 3. Wait for all processors to reach this point.
>> 4. Enter the no-fill cache mode. (Set the CD flag in control register CR0 to 1
>>     and the NW flag to 0.)
>> 5. Flush all caches using the WBINVD instructions. Note on a processor that
>>     supports self-snooping, CPUID feature flag bit 27, this step is unnecessary.
>> 6. If the PGE flag is set in control register CR4, flush all TLBs by clearing
>>     that flag.
>> 7. If the PGE flag is clear in control register CR4, flush all TLBs by executing
>>     a MOV from control register CR3 to another register and then a MOV from that
>>     register back to CR3.
>> 8. Disable all range registers (by clearing the E flag in register MTRRdefType).
>>     If only variable ranges are being modified, software may clear the valid bits
>>     for the affected register pairs instead.
>> 9. Update the MTRRs.
>> 10. Enable all range registers (by setting the E flag in register MTRRdefType).
>>      If only variable-range registers were modified and their individual valid
>>      bits were cleared, then set the valid bits for the affected ranges instead.
>> 11. Flush all caches and all TLBs a second time. (The TLB flush is required for
>>      Pentium 4, Intel Xeon, and P6 family processors. Executing the WBINVD
>>      instruction is not needed when using Pentium 4, Intel Xeon, and P6 family
>>      processors, but it may be needed in future systems.)
>> 12. Enter the normal cache mode to re-enable caching. (Set the CD and NW flags
>>      in control register CR0 to 0.)
>> 13. Set PGE flag in control register CR4, if cleared in Step 6 (above).
>> 14. Wait for all processors to reach this point.
>> 15. Enable interrupts.
>>
>> So cache disabling is recommended.
> 
> Yeah, I read that.
> 
> But the question is what kind of scenario cache disabling is actually
> prevents if self-snoop is supported? In this case cache stays intact (no
> WBINVD). The next time a cache line gets accessed with different caching
> mode the old line gets snooped, right?
> 
> Would it be valid to avoid touching CR0.CD if X86_FEATURE_SELFSNOOP?
> 

That's a question for the Intel architects, I guess.

I'd just ask them how to setup PAT in TDX guests. Either they need to
change the recommended setup sequence, or the PAT support bit needs to
be cleared IMO.


Juergen

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