Re: [PATCH RFC v7 14/64] x86/sev: Add the host SEV-SNP initialization support

[Borislav Petkov]

On Wed, Dec 14, 2022 at 01:40:06PM -0600, Michael Roth wrote:
> From: Brijesh Singh <brijesh.singh@xxxxxxx>
> 
> The memory integrity guarantees of SEV-SNP are enforced through a new
> structure called the Reverse Map Table (RMP). The RMP is a single data
> structure shared across the system that contains one entry for every 4K
> page of DRAM that may be used by SEV-SNP VMs. The goal of RMP is to
> track the owner of each page of memory. Pages of memory can be owned by
> the hypervisor, owned by a specific VM or owned by the AMD-SP. See APM2
> section 15.36.3 for more detail on RMP.
> 
> The RMP table is used to enforce access control to memory. The table itself
> is not directly writable by the software. New CPU instructions (RMPUPDATE,
> PVALIDATE, RMPADJUST) are used to manipulate the RMP entries.
> 
> Based on the platform configuration, the BIOS reserves the memory used
> for the RMP table. The start and end address of the RMP table must be
> queried by reading the RMP_BASE and RMP_END MSRs. If the RMP_BASE and
> RMP_END are not set then disable the SEV-SNP feature.
> 
> The SEV-SNP feature is enabled only after the RMP table is successfully
> initialized.
> 
> Also set SYSCFG.MFMD when enabling SNP as SEV-SNP FW >= 1.51 requires
> that SYSCFG.MFMD must be se

			   set.
> 
> RMP table entry format is non-architectural and it can vary by processor
> and is defined by the PPR. Restrict SNP support on the known CPU model
> and family for which the RMP table entry format is currently defined for.
> 
> Signed-off-by: Brijesh Singh <brijesh.singh@xxxxxxx>
> Signed-off-b: Ashish Kalra <ashish.kalra@xxxxxxx>
	     ^^

Somebody ate a 'y' here. :)

> Signed-off-by: Michael Roth <michael.roth@xxxxxxx>
> ---
>  arch/x86/include/asm/disabled-features.h |   8 +-
>  arch/x86/include/asm/msr-index.h         |  11 +-
>  arch/x86/kernel/sev.c                    | 180 +++++++++++++++++++++++
>  3 files changed, 197 insertions(+), 2 deletions(-)

...

> +static __init int __snp_rmptable_init(void)

Why is this one carved out of snp_rmptable_init() ?

> +{
> +	u64 rmp_base, sz;
> +	void *start;
> +	u64 val;
> +
> +	if (!get_rmptable_info(&rmp_base, &sz))
> +		return 1;
> +
> +	start = memremap(rmp_base, sz, MEMREMAP_WB);
> +	if (!start) {
> +		pr_err("Failed to map RMP table addr 0x%llx size 0x%llx\n", rmp_base, sz);
> +		return 1;
> +	}
> +
> +	/*
> +	 * Check if SEV-SNP is already enabled, this can happen in case of
> +	 * kexec boot.
> +	 */
> +	rdmsrl(MSR_AMD64_SYSCFG, val);
> +	if (val & MSR_AMD64_SYSCFG_SNP_EN)
> +		goto skip_enable;
> +
> +	/* Initialize the RMP table to zero */

Useless comment.

> +	memset(start, 0, sz);
> +
> +	/* Flush the caches to ensure that data is written before SNP is enabled. */
> +	wbinvd_on_all_cpus();
> +
> +	/* MFDM must be enabled on all the CPUs prior to enabling SNP. */
> +	on_each_cpu(mfd_enable, NULL, 1);
> +
> +	/* Enable SNP on all CPUs. */
> +	on_each_cpu(snp_enable, NULL, 1);

What happens if someone boots the machine with maxcpus=N, where N is
less than all CPUs on the machine? The hotplug notifier should handle it
but have you checked that it works fine?

> +skip_enable:
> +	rmptable_start = (unsigned long)start;
> +	rmptable_end = rmptable_start + sz - 1;
> +
> +	return 0;
> +}
> +
> +static int __init snp_rmptable_init(void)
> +{
> +	int family, model;
> +
> +	if (!cpu_feature_enabled(X86_FEATURE_SEV_SNP))
> +		return 0;
> +
> +	family = boot_cpu_data.x86;
> +	model  = boot_cpu_data.x86_model;

Looks useless - just use boot_cpu_data directly below.

> +
> +	/*
> +	 * 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 (family != 0x19 || model > 0xaf)
> +		goto nosnp;
> +
> +	if (amd_iommu_snp_enable())
> +		goto nosnp;
> +
> +	if (__snp_rmptable_init())
> +		goto nosnp;
> +
> +	cpuhp_setup_state(CPUHP_AP_ONLINE_DYN, "x86/rmptable_init:online", __snp_enable, NULL);
> +
> +	return 0;
> +
> +nosnp:
> +	setup_clear_cpu_cap(X86_FEATURE_SEV_SNP);
> +	return -ENOSYS;
> +}

Thx.

-- 
Regards/Gruss,
    Boris.

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