linux-kernel - Re: [PATCH v7 01/10] KVM: SEV: Disable SEV-SNP support on initialization failure

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Message-ID: <6c497385-6447-4d81-8d93-20c46cad29f9@amd.com>
Date: Tue, 25 Feb 2025 13:45:04 -0600
From: "Pratik R. Sampat" <prsampat@....com>
To: Tom Lendacky <thomas.lendacky@....com>, <linux-kernel@...r.kernel.org>,
	<x86@...nel.org>, <kvm@...r.kernel.org>, <linux-crypto@...r.kernel.org>,
	<linux-kselftest@...r.kernel.org>
CC: <seanjc@...gle.com>, <pbonzini@...hat.com>, <tglx@...utronix.de>,
	<mingo@...hat.com>, <bp@...en8.de>, <dave.hansen@...ux.intel.com>,
	<shuah@...nel.org>, <pgonda@...gle.com>, <ashish.kalra@....com>,
	<nikunj@....com>, <pankaj.gupta@....com>, <michael.roth@....com>,
	<sraithal@....com>
Subject: Re: [PATCH v7 01/10] KVM: SEV: Disable SEV-SNP support on
 initialization failure



On 2/25/2025 1:09 PM, Tom Lendacky wrote:
> On 2/25/25 11:45, Pratik R. Sampat wrote:
>> On 2/25/2025 10:41 AM, Pratik R. Sampat wrote:
>>> Hi Tom,
>>>
>>> On 2/24/2025 3:28 PM, Tom Lendacky wrote:
>>>> On 2/21/25 15:01, Pratik R. Sampat wrote:
>>>>> During platform init, SNP initialization may fail for several reasons,
>>>>> such as firmware command failures and incompatible versions. However,
>>>>> the KVM capability may continue to advertise support for it. Export this
>>>>> information to KVM and withdraw SEV-SNP support if has not been
>>>>> successfully initialized.
>>>>
>>>> Hmmm... rather than creating a new API, can you just issue an
>>>> SNP_PLATFORM_STATUS command and see if the SNP is not in the UNINIT state?
>>>>
>>>
>>> Although reading sev->snp_initialized is probably cheaper to do, it is
>>> cleaner to query the platform status.
>>>
>>> Querying SNP_PLATFORM_STATUS requires the pages to transition to
>>> firmware-owned and back, and the helpers for it are implemented within
>>> sev-dev.c. So, similar to sev_platform_status(), I'm thinking it is
>>> probably better to create the snp_platform_status() API as well and use
>>> that within KVM to check the state.
>>>
>>
>> Although I am guessing the initial intent was to not have an API exposed
>> at all from CCP and only make the SNP_PLATFORM_STATUS call instead?
>>
>> Since that may not be cleanly possible (we have helpers for page state
>> conversions such as rmp_mark_pages_firmware() in ccp) without
>> duplicating functionality in KVM as well, I guess the question really
>> boils down to whether we export the cheaper snp_initialized() or the
>> snp_platform_status() API instead?
> 
> Taking a closer look, we do already have APIs that KVM uses to allocate
> firmware pages (output pages for SNP APIs) that can be used:
> snp_alloc_firmware_page() and snp_free_firmware_page().
> 
> I think that should be enough to use sev_do_cmd() to perform the
> SEV_CMD_SNP_PLATFORM_STATUS command without exposing a new API.
> 

Ah, I had missed that! Calling the SNP_PLATFORM_STATUS this way works.

Pratik

> Thanks,
> Tom
> 
>>
>> Thanks again!
>> Pratik