linux-kernel - Re: [PATCH v4 05/15] x86/sev: Use kernel provided SVSM Calling Areas

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Date: Wed, 8 May 2024 15:09:02 -0500
From: Tom Lendacky <thomas.lendacky@....com>
To: Borislav Petkov <bp@...en8.de>
Cc: linux-kernel@...r.kernel.org, x86@...nel.org, linux-coco@...ts.linux.dev,
 svsm-devel@...onut-svsm.dev, Thomas Gleixner <tglx@...utronix.de>,
 Ingo Molnar <mingo@...hat.com>, Dave Hansen <dave.hansen@...ux.intel.com>,
 "H. Peter Anvin" <hpa@...or.com>, Andy Lutomirski <luto@...nel.org>,
 Peter Zijlstra <peterz@...radead.org>,
 Dan Williams <dan.j.williams@...el.com>, Michael Roth
 <michael.roth@....com>, Ashish Kalra <ashish.kalra@....com>
Subject: Re: [PATCH v4 05/15] x86/sev: Use kernel provided SVSM Calling Areas

On 5/8/24 14:58, Borislav Petkov wrote:
> On Wed, May 08, 2024 at 02:13:17PM -0500, Tom Lendacky wrote:
>> ok, maybe __perform_svsm_msr_protocol or such.
> 
> We'll bikeshed it in the coming weeks.

:)

> 
>> There's quite a bit of interaction so I'll make sure to prefix everything.
> 
> Ack.
> 
>> The paravirt versions of local_irq_save and local_irq_restore can't be used
>> as early as this routine is called.
> 
> tglx says you should do native_local_irq_save()/.._restore() helpers
> just like the arch_local_irq_save()/..._restore() ones but use only
> native_ functions without the paravirt gunk.
> 
> In a prepatch pls.

Will do.

> 
>>>> +	struct svsm_call call = {};
>>>> +	int ret;
>>>> +	u64 pa;
>>>> +
>>>> +	/*
>>>> +	 * Record the SVSM Calling Area address (CAA) if the guest is not
>>>> +	 * running at VMPL0. The CA will be used to communicate with the
>>>> +	 * SVSM to perform the SVSM services.
>>>> +	 */
>>>> +	setup_svsm_ca(cc_info);
>>>> +
>>>> +	/* Nothing to do if not running under an SVSM. */
>>>> +	if (!vmpl)
>>>> +		return;
>>>
>>> You set up stuff above and now you bail out?
>>
>> setup_svsm_ca() is what sets the vmpl variable. So nothing will have been
>> setup if the VMPL is zero, in which case we don't continue on.
> 
> You still assign
> 
>          /*
>           * The CA is identity mapped when this routine is called, both by the
>           * decompressor code and the early kernel code.
>           */
>          boot_svsm_caa = (struct svsm_ca *)caa;
>          boot_svsm_caa_pa = caa;
> 
> regardless of vmpl.

If we're not running at VMPL0 (based on the RMPADJUST check) and if the 
SVSM doesn't advertise a non-zero VMPL value, we will self-terminate. So 
those values are only set if we are not running at VMPL0 and the SVSM 
has provided a non-zero value to us.

I'm going to turn the function into a bool function so that the call 
becomes:

	if (!svsm_setup_caa(cc_info))
		return;

> 
> I think you should assign those only when vmpl != 0.

I do. I think you're missing the RMPADJUST check that causes the 
function to return early if we're running at VMPL0.

> 
> Otherwise the code is confusing.
> 
>>> Judging by setup_svsm_ca() you don't really need that vmpl var but you
>>> can check
>>>
>>> 	if (!boot_svsm_caa)
>>> 		return;
>>>
>>> to determine whether a SVSM was detected.
>>
>> Yes, but the vmpl var will be used for attestation requests, sysfs, etc.
> 
> I guess that comes later in the patchset...
> 
>>> Huh, setup_svsm_ca() already assigned those...
>>
>> setup_svsm_ca() assigned the ones from the secrets page. The kernel now
>> switches to using its own CA.
> 
> Comment pls.

There's a block comment above it all, but maybe it isn't clear enough. 
I'll rework it.

Thanks,
Tom

> 
> Thx.
>