linux-kernel - Re: [PATCH v3 0/5] efi: Don't initalize SEV-SNP from the EFI stub

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Message-ID: <aea21a34-e393-0f50-006d-d88b3a4a7a94@amd.com>
Date: Fri, 25 Apr 2025 13:40:49 -0500
From: Tom Lendacky <thomas.lendacky@....com>
To: Ard Biesheuvel <ardb@...nel.org>, Ard Biesheuvel <ardb@...gle.com>
Cc: Ard Biesheuvel <ardb+git@...gle.com>, linux-efi@...r.kernel.org,
 x86@...nel.org, linux-kernel@...r.kernel.org, mingo@...nel.org,
 Borislav Petkov <bp@...en8.de>, Dionna Amalie Glaze
 <dionnaglaze@...gle.com>, Kevin Loughlin <kevinloughlin@...gle.com>
Subject: Re: [PATCH v3 0/5] efi: Don't initalize SEV-SNP from the EFI stub

On 4/25/25 13:18, Tom Lendacky wrote:
> On 4/24/25 09:18, Tom Lendacky wrote:
>> On 4/24/25 02:22, Ard Biesheuvel wrote:
>>> On Tue, 22 Apr 2025 at 18:40, Ard Biesheuvel <ardb@...gle.com> wrote:
>>
>> Let me take a look...
> 
> Initial look at something like this works (along with the fix for the
> mistake I made in OVMF). I need to test the kexec path to be certain,
> though.

Ah, this version doesn't have the part in arch/x86/include/asm/sev.h that
declares snp_vmpl_checked as an extern. But other than that...

Thanks,
Tom

> 
> diff --git a/arch/x86/boot/compressed/sev.c b/arch/x86/boot/compressed/sev.c
> index 478c65149cf0..d2f9cbbe943b 100644
> --- a/arch/x86/boot/compressed/sev.c
> +++ b/arch/x86/boot/compressed/sev.c
> @@ -142,6 +142,7 @@ u64 svsm_get_caa_pa(void)
>  int svsm_perform_call_protocol(struct svsm_call *call);
>  
>  u8 snp_vmpl;
> +bool snp_vmpl_checked;
>  
>  /* Include code for early handlers */
>  #include "../../boot/startup/sev-shared.c"
> @@ -241,6 +242,29 @@ static bool early_setup_ghcb(void)
>  
>  void snp_accept_memory(phys_addr_t start, phys_addr_t end)
>  {
> +	if (!snp_vmpl_checked) {
> +		unsigned int eax, ebx, ecx, edx;
> +
> +		/*
> +		 * CPUID Fn8000_001F_EAX[28] - SVSM support
> +		 */
> +		eax = 0x8000001f;
> +		ecx = 0;
> +		native_cpuid(&eax, &ebx, &ecx, &edx);
> +		if (eax & BIT(28)) {
> +			struct msr m;
> +
> +			/* Obtain the address of the calling area to use */
> +			boot_rdmsr(MSR_SVSM_CAA, &m);
> +			boot_svsm_caa = (void *)m.q;
> +			boot_svsm_caa_pa = m.q;
> +
> +			snp_vmpl = 2;
> +		}
> +
> +		snp_vmpl_checked = true;
> +	}
> +
>  	for (phys_addr_t pa = start; pa < end; pa += PAGE_SIZE)
>  		__page_state_change(pa, SNP_PAGE_STATE_PRIVATE);
>  }
> diff --git a/arch/x86/boot/startup/sev-shared.c b/arch/x86/boot/startup/sev-shared.c
> index 173f3d1f777a..5cca01700280 100644
> --- a/arch/x86/boot/startup/sev-shared.c
> +++ b/arch/x86/boot/startup/sev-shared.c
> @@ -1342,6 +1342,8 @@ static bool __head svsm_setup_ca(const struct cc_blob_sev_info *cc_info)
>  
>  	BUILD_BUG_ON(sizeof(*secrets_page) != PAGE_SIZE);
>  
> +	snp_vmpl_checked = true;
> +
>  	/*
>  	 * Check if running at VMPL0.
>  	 *
> diff --git a/arch/x86/coco/sev/core.c b/arch/x86/coco/sev/core.c
> index 5b145446e991..5011b3a93a21 100644
> --- a/arch/x86/coco/sev/core.c
> +++ b/arch/x86/coco/sev/core.c
> @@ -99,6 +99,7 @@ DEFINE_PER_CPU(struct sev_es_save_area *, sev_vmsa);
>   */
>  u8 snp_vmpl __ro_after_init;
>  EXPORT_SYMBOL_GPL(snp_vmpl);
> +bool snp_vmpl_checked __ro_after_init;
>  
>  static u64 __init get_snp_jump_table_addr(void)
>  {
> 
>>
>> Thanks,
>> Tom