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

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Date: Mon, 19 Feb 2024 16:09:47 -0600
From: Tom Lendacky <thomas.lendacky@....com>
To: Borislav Petkov <bp@...en8.de>
Cc: "Huang, Kai" <kai.huang@...el.com>, linux-kernel@...r.kernel.org,
 x86@...nel.org, dave.hansen@...el.com, kirill.shutemov@...ux.intel.com,
 tglx@...utronix.de, mingo@...hat.com, hpa@...or.com, luto@...nel.org,
 peterz@...radead.org, chao.gao@...el.com, bhe@...hat.com,
 nik.borisov@...e.com, pbonzini@...hat.com
Subject: Re: [PATCH 1/4] x86/coco: Add a new CC attribute to unify cache flush
 during kexec

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.

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

> 
> Any downsides to that which are actually real and noticeable?
> 
> Thx.
>