Re: [PATCH v10 06/50] x86/sev: Add the host SEV-SNP initialization support

[Jeremi Piotrowski <jpiotrowski@linux.microsoft.com>]

On 08/11/2023 07:14, Borislav Petkov wrote:
> On Tue, Nov 07, 2023 at 04:33:41PM -0600, Kalra, Ashish wrote:
>> We will still need some method to tell the IOMMU driver if SNP
>> support/feature is disabled by this function, for example, when CPU family
>> and model is not supported by SNP and we jump to no_snp label.
> 
> See below.
> 
>> The reliable way for this to work is to ensure snp_rmptable_init() is called
>> before IOMMU initialization and then IOMMU initialization depends on SNP
>> feature flag setup by snp_rmptable_init() to enable SNP support on IOMMU or
>> not.
> 
> Yes, this whole SNP initialization needs to be reworked and split this
> way:

I agree with Borislav and have some comments of my own.

> 
> - early detection work which needs to be done once goes to
>   bsp_init_amd(): that's basically your early_detect_mem_encrypt() stuff
>   which needs to happen exactly only once and early.
> 
> - Any work like:
> 
> 	 c->x86_phys_bits -= (cpuid_ebx(0x8000001f) >> 6) & 0x3f;
> 
>   and the like which needs to happen on each AP, gets put in a function
>   which gets called by init_amd().
> 
> By the time IOMMU gets to init, you already know whether it should
> enable SNP and check X86_FEATURE_SEV_SNP.

This flow would suit me better too. In SNP-host capable Hyper-V VMs there
is no IOMMU and I've had to resort to early return from amd_iommu_snp_enable()
to prevent it from disabling SNP [1]. In addition to what Borislav posted,
you'd just need to enforce that if IOMMU is detected it actually gets enabled.

[1]: https://lore.kernel.org/lkml/20230213103402.1189285-6-jpiotrowski@linux.microsoft.com/

> 
> Finally, you call __snp_rmptable_init() which does the *per-CPU* init
> work which is still pending.

Yes please, and the only rmp thing left to do per-CPU would be to check that the MSRs are
set the same as the value read from CPU0.

Running the early_rmptable_check() from early_detect_mem_encrypt() and on every CPU
makes it difficult to support a kernel allocated RMP table. If you look at what I did
for the mentioned Hyper-V SNP-host VMs [2] (which I think is reasonable) the RMP table
is allocated in init_mem_mapping() and the addresses are propagated to other CPUs through 
hv_cpu_init(), which is called from cpuhp_setup_state(CPUHP_AP_HYPERV_ONLINE, ...). So
it would be great if any init works plays nice with cpu hotplug notifiers.

[2]: https://lore.kernel.org/lkml/20230213103402.1189285-2-jpiotrowski@linux.microsoft.com/

Thanks,
Jeremi

> 
> Ok?
> 
> Ontop of the previous ontop patch:
> 
> ---
> diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c
> index 6cc2074fcea3..a9c95e5d6b06 100644
> --- a/arch/x86/kernel/cpu/amd.c
> +++ b/arch/x86/kernel/cpu/amd.c
> @@ -674,8 +674,19 @@ static void early_detect_mem_encrypt(struct cpuinfo_x86 *c)
>  		if (!(msr & MSR_K7_HWCR_SMMLOCK))
>  			goto clear_sev;
>  
> -		if (cpu_has(c, X86_FEATURE_SEV_SNP) && !early_rmptable_check())
> -			goto clear_snp;
> +		if (cpu_has(c, X86_FEATURE_SEV_SNP)) {
> +			/*
> +			 * RMP table entry format is not architectural and it can vary by processor
> +			 * and is defined by the per-processor PPR. Restrict SNP support on the known
> +			 * CPU model and family for which the RMP table entry format is currently
> +			 * defined for.
> +			 */
> +			if (c->x86 != 0x19 || c->x86_model > 0xaf)
> +				goto clear_snp;
> +
> +			if (!early_rmptable_check())
> +				goto clear_snp;
> +		}
>  
>  		return;
>  
> diff --git a/arch/x86/virt/svm/sev.c b/arch/x86/virt/svm/sev.c
> index 9237c327ad6d..5a71df9ae4cb 100644
> --- a/arch/x86/virt/svm/sev.c
> +++ b/arch/x86/virt/svm/sev.c
> @@ -199,14 +199,6 @@ static int __init snp_rmptable_init(void)
>  	if (!amd_iommu_snp_en)
>  		return 0;
>  
> -	/*
> -	 * RMP table entry format is not architectural and it can vary by processor and
> -	 * is defined by the per-processor PPR. Restrict SNP support on the known CPU
> -	 * model and family for which the RMP table entry format is currently defined for.
> -	 */
> -	if (boot_cpu_data.x86 != 0x19 || boot_cpu_data.x86_model > 0xaf)
> -		goto nosnp;
> -
>  	if (__snp_rmptable_init())
>  		goto nosnp;
>