linux-kernel - Re: [PATCH 1/4] x86/coco: Add a new CC attribute to unify cache flush during kexec

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Date: Tue, 20 Feb 2024 08:40:05 -0600
From: Tom Lendacky <thomas.lendacky@....com>
To: "Huang, Kai" <kai.huang@...el.com>, "bp@...en8.de" <bp@...en8.de>
Cc: "Gao, Chao" <chao.gao@...el.com>, "Hansen, Dave" <dave.hansen@...el.com>,
 "luto@...nel.org" <luto@...nel.org>, "x86@...nel.org" <x86@...nel.org>,
 "peterz@...radead.org" <peterz@...radead.org>, "hpa@...or.com"
 <hpa@...or.com>, "mingo@...hat.com" <mingo@...hat.com>,
 "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
 "tglx@...utronix.de" <tglx@...utronix.de>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 "pbonzini@...hat.com" <pbonzini@...hat.com>,
 "nik.borisov@...e.com" <nik.borisov@...e.com>,
 "bhe@...hat.com" <bhe@...hat.com>
Subject: Re: [PATCH 1/4] x86/coco: Add a new CC attribute to unify cache flush
 during kexec

On 2/19/24 20:57, Huang, Kai wrote:
> On Mon, 2024-02-19 at 16:09 -0600, Tom Lendacky wrote:
>> On 2/19/24 14:32, Borislav Petkov wrote:
>>> On Mon, Feb 19, 2024 at 01:45:37PM -0600, Tom Lendacky wrote:
>>>> This change won't return the correct answer. The check needs to remain
>>>> against the sev_status value.
>>>
>>> Feel free to explain because this patch is confusing me.
>>
>> In your previous email, you want to put the CC_ATTR_HOST_MEM_INCOHERENT
>> case statement with the CC_ATTR_MEM_ENCRYPT case which is returning
>> sme_me_mask. That will be zero/false if SME is not active, skipping the
>> WBINVD. But, in reality you still need to perform the WBINVD in case the
>> kexec target is doing mem_encrypt=on.
>>
>> That's why the '!(sev_status & MSR_AMD64_SEV_ENABLED)' works here.
>> Basically, if you are bare-metal, it will return true. And it will only
>> return true for machines that support SME and have the
>> MSR_AMD64_SYSCFG_MEM_ENCRYPT bit set in SYS_CFG MSR because of where the
>> 'cc_vendor = CC_VENDOR_AMD' assignment is.
>>
> 
> [...]
> 
>> However, if you move the
>> 'cc_vendor = CC_VENDOR_AMD' to before the if statement, then you will have
>> the WBINVD called for any machine that supports SME, even if SME is not
>> possible because the proper bit in the SYS_CFG MSR hasn't been set.
> 
> Hi Tom,
> 
> Thanks for clarifying.  However it seems to me that this is the behaviour in the
> current upstream code.  The stop_this_cpu() checks CPUID directly w/o checking
> the SYS_CFG MSR:

Correct, it will match the upstream behavior this way. It would have been 
improved slightly with your original patch by avoiding the WBINVD if the 
MSR_AMD64_SYSCFG_MEM_ENCRYPT wasn't set.

> 
>          if (c->extended_cpuid_level >= 0x8000001f &&
> 			(cpuid_eax(0x8000001f) & BIT(0)))
>                  native_wbinvd();
> 
> I believe the BIT(0) in CPUID, which is "Secure Memory Encryption Support", will
> always be true regardless of whether the MSR_AMD64_SYSCFG_MEM_ENCRYPT is set in
> SYS_CFG MSR?
> 
> If so, IIUC moving the 'cc_vendor = CC_VENDOR_AMD' to the place right before the
> if statement as suggested by Boris seems just follows the current behaviour in
> the upstream code.

Yep, that's how I see it, too.

Thanks,
Tom

> 
> Of course we need to always return true for CC_ATTR_HOST_MEM_INCOHERENT but not
> querying sme_me_mask.
> 
>>
>> I know what I'm trying to say, let me know if it is making sense...
>>
>>>
>>>>> So you can't put it before the if - just slap it in both branches. Geez!
>>>>
>>>> I think that will still work because sme_me_mask and sev_status will both be
>>>> 0 on bare-metal if 'msr & MSR_AMD64_SYSCFG_MEM_ENCRYPT' doesn't evaluate to
>>>> true. However, that will cause any platform that hasn't enabled memory
>>>> encryption (see SYS_CFG MSR), to also perform the WBINVD.
>>>
>>> If it keeps the code simpler I don't mind. That's so not a fast path.
>>>
>>>> That won't work, because the current system may not have SME active. The
>>>> cases that needs to be caught are kexec'ing from a mem_encrypt=off to a
>>>> mem_encrypt=on or from a mem_encrypt=on to a mem_encrypt=off.
>>>
>>> And I'm saying, we should keep it simple and simply WBINVD on SME
>>> capable machines, regardless of the encryption setting.
>>
>> In that case, CC_ATTR_HOST_MEM_INCOHERENT needs to be separate from
>> CC_ATTR_MEM_ENCRYPT as the original patch has it. The comment might make
>> more sense as:
>>
>> 	 * CC_ATTR_HOST_MEM_INCOHERENT represents whether SME is possible
>> 	 * on the platform, regardless of whether mem_encrypt=on has been
>> 	 * used to make SME active.
>>
>> Thanks,
>> Tom
> 
> This looks good to me.  Thanks!
> 
>