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Date: Tue, 27 Feb 2024 09:11:00 -0600
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Subject: Re: [PATCH v6 05/10] x86/sme: Avoid SME/SVE related checks on
 non-SME/SVE platforms
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To: Ard Biesheuvel <ardb@kernel.org>
Cc: Ard Biesheuvel <ardb+git@google.com>, linux-kernel@vger.kernel.org,
 Kevin Loughlin <kevinloughlin@google.com>,
 Dionna Glaze <dionnaglaze@google.com>, Thomas Gleixner <tglx@linutronix.de>,
 Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
 Dave Hansen <dave.hansen@linux.intel.com>, Andy Lutomirski
 <luto@kernel.org>, Brian Gerst <brgerst@gmail.com>
References: <20240226142952.64769-12-ardb+git@google.com>
 <20240226142952.64769-17-ardb+git@google.com>
 <229aaa22-5e69-4f18-b5f7-fec83109bb53@amd.com>
 <CAMj1kXGRTO=HupSKtGhpCpMt4nNpQLzuAUzRj8Qvdfnc2E=W5g@mail.gmail.com>
From: Tom Lendacky <thomas.lendacky@amd.com>
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On 2/27/24 08:55, Ard Biesheuvel wrote:
> On Mon, 26 Feb 2024 at 22:38, Tom Lendacky <thomas.lendacky@amd.com> wrote:
>>
>> On 2/26/24 08:29, Ard Biesheuvel wrote:
>>> From: Ard Biesheuvel <ardb@kernel.org>
>>>

>>> -void __init sme_enable(struct boot_params *bp)
>>> +void __head sme_enable(struct boot_params *bp)
>>>    {
>>>        const char *cmdline_ptr, *cmdline_arg, *cmdline_on;
>>>        unsigned int eax, ebx, ecx, edx;
>>>        unsigned long feature_mask;
>>>        unsigned long me_mask;
>>>        char buffer[16];
>>> -     bool snp;
>>>        u64 msr;
>>>
>>> -     snp = snp_init(bp);
>>
>> The snp_init() call is here because the SNP CPUID table needs to be
>> established before the below CPUID instruction is executed. This can't be
>> moved.
>>
> 
> Yeah, good point. I didn't spot this in my SEV-SNP boot testing,
> presumably because the firmware's VC handler is still active in my
> case, but this isn't guaranteed, e.g., when booting via the
> decompressor, or when using 5-level paging.

Actually, the kernel's do_vc_no_ghcb() is the handler that gets invoked. 
Since the CPUID table is not initialized, -EOPNOTSUPP is returned from 
snp_cpuid(), which results in the handler using the MSR protocol to get 
the CPUID information from the hypervisor - which we don't want for SNP in 
this situation.

Thanks,
Tom


> 
> Thanks for the review.