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To:     Borislav Petkov <bp@alien8.de>
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Subject: RE: [PATCH Part2 v6 03/49] x86/sev: Add the host SEV-SNP
 initialization support
Thread-Topic: [PATCH Part2 v6 03/49] x86/sev: Add the host SEV-SNP
 initialization support
Thread-Index: AQHYmcRAcxQqbwBTdUCpGH+ovKgAp62Ei1rQ
Date:   Tue, 19 Jul 2022 03:56:25 +0000
Message-ID: <SN6PR12MB27674548A8C8ACF5E53C0DB78E8F9@SN6PR12MB2767.namprd12.prod.outlook.com>
References: <cover.1655761627.git.ashish.kalra@amd.com>
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[AMD Official Use Only - General]

Hello Boris,

>> +	 * See PPR Family 19h Model 01h, Revision B1 section 2.1.4.2 for more
>> +	 * information on memory requirement.

>That section number will change over time - if you want to refer to some s=
ection just use its title so that people can at least grep for the relevant=
 text.

This will all go into sev.c, instead of the header file, as this is non-arc=
hitectural and per-processor and the structure won't be exposed to the rest
of the kernel.  The above PPR reference and potentially in future an archit=
ectural method of reading the RMP table entries will be moved into it.

>> +	 */
>> +	nr_pages =3D totalram_pages();
>> +	calc_rmp_sz =3D (((rmp_sz >> PAGE_SHIFT) + nr_pages) << 4) +=20
>> +RMPTABLE_CPU_BOOKKEEPING_SZ;

>use totalram_pages() directly and get rid of nr_pages.
Ok.

>> +	 * kexec boot.
>> +	 */
>> +	rdmsrl(MSR_AMD64_SYSCFG, val);
>> +	if (val & MSR_AMD64_SYSCFG_SNP_EN)
>> +		goto skip_enable;
>> +
>> +	/* Initialize the RMP table to zero */
>> +	memset(start, 0, sz);

>Do I understand it correctly that in the kexec case the second, kexec-ed k=
ernel is reusing the previous kernel's RMP table so it should not be cleare=
d?
I believe that with kexec and after issuing the shutdown command, the RMP t=
able needs to be fully initialized, so we should be re-initializing the RMP
table to zero here.

>>
>> +
>> +static int __init snp_rmptable_init(void) {
>> +	if (!boot_cpu_has(X86_FEATURE_SEV_SNP))

>cpu_feature_enabled
Ok.

>> +		return 0;
>> +
>> +	if (!iommu_sev_snp_supported())
>> +		goto nosnp;
>> +
>> +	if (__snp_rmptable_init())
>> +		goto nosnp;
>> +
>> +	cpuhp_setup_state(CPUHP_AP_ONLINE_DYN, "x86/rmptable_init:online",=20
>> +__snp_enable, NULL);
>> +
>> +	return 0;
>> +
>> +nosnp:
>> +	setup_clear_cpu_cap(X86_FEATURE_SEV_SNP);
>> +	return 1;
>> +}
>> +
>> +/*
>> + * This must be called after the PCI subsystem. This is because=20
>> +before enabling
>> + * the SNP feature we need to ensure that IOMMU supports the SEV-SNP fe=
ature.
>> + * The iommu_sev_snp_support() is used for checking the feature, and=20
>> +it is
>> + * available after subsys_initcall().

>I'd much more appreciate here a short formulation explaining why is IOMMU =
needed for SNP rather than the obvious.

Yes, IOMMU is enforced for SNP to ensure that HV cannot program DMA directl=
y into guest private memory. In case of SNP,
the IOMMU makes sure that the page(s) used for DMA are HV owned.

Thanks,
Ashish