Re: [PATCH Part2 v6 03/49] x86/sev: Add the host SEV-SNP initialization support

[Borislav Petkov]

On Mon, Jun 20, 2022 at 11:02:01PM +0000, Ashish Kalra wrote:
> +/*
> + * The first 16KB from the RMP_BASE is used by the processor for the
> + * bookkeeping, the range need to be added during the RMP entry lookup.

needs

> +static int __snp_enable(unsigned int cpu)
> +{
> +	u64 val;
> +
> +	if (!cpu_feature_enabled(X86_FEATURE_SEV_SNP))
> +		return 0;
> +
> +	rdmsrl(MSR_AMD64_SYSCFG, val);
> +
> +	val |= MSR_AMD64_SYSCFG_SNP_EN;
> +	val |= MSR_AMD64_SYSCFG_SNP_VMPL_EN;
> +
> +	wrmsrl(MSR_AMD64_SYSCFG, val);
> +
> +	return 0;
> +}
> +
> +static __init void snp_enable(void *arg)
> +{
> +	__snp_enable(smp_processor_id());
> +}

Get rid of that silly wrapper - you're not even using that @cpu argument.

> +static bool get_rmptable_info(u64 *start, u64 *len)
> +{
> +	u64 calc_rmp_sz, rmp_sz, rmp_base, rmp_end, nr_pages;
> +
> +	rdmsrl(MSR_AMD64_RMP_BASE, rmp_base);
> +	rdmsrl(MSR_AMD64_RMP_END, rmp_end);
> +
> +	if (!rmp_base || !rmp_end) {
> +		pr_info("Memory for the RMP table has not been reserved by BIOS\n");

pr_err

> +		return false;
> +	}
> +
> +	rmp_sz = rmp_end - rmp_base + 1;
> +
> +	/*
> +	 * Calculate the amount the memory that must be reserved by the BIOS to
> +	 * address the full system RAM. The reserved memory should also cover the

"... address the whole RAM."

> +	 * RMP table itself.
> +	 *
> +	 * 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
section just use its title so that people can at least grep for the
relevant text.

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

use totalram_pages() directly and get rid of nr_pages.

> +
> +	if (calc_rmp_sz > rmp_sz) {
> +		pr_info("Memory reserved for the RMP table does not cover full system RAM (expected 0x%llx got 0x%llx)\n",
> +			calc_rmp_sz, rmp_sz);

pr_err

> +		return false;
> +	}
> +
> +	*start = rmp_base;
> +	*len = rmp_sz;
> +
> +	pr_info("RMP table physical address 0x%016llx - 0x%016llx\n", rmp_base, rmp_end);

"RMP table physical address range: ...[0x.. - 0x..]"

> +
> +	return true;
> +}
> +
> +static __init int __snp_rmptable_init(void)

s/int/bool/

> +{
> +	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 0x%llx+0x%llx\n", rmp_base, sz);
							^^^^^^

either write the size in decimal or do a normal interval.

> +		return 1;
> +	}
> +
> +	/*
> +	 * Check if SEV-SNP is already enabled, this can happen if we are coming from

Who is "we"?

Pls get rid of all "we" in the comments and use passive formulations.

> +	 * 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
kernel is reusing the previous kernel's RMP table so it should not be
cleared?

> +
> +	/* Flush the caches to ensure that data is written before SNP is enabled. */
> +	wbinvd_on_all_cpus();
> +
> +	/* Enable SNP on all CPUs. */
> +	on_each_cpu(snp_enable, NULL, 1);
> +
> +skip_enable:
> +	rmptable_start = (unsigned long)start;
> +	rmptable_end = rmptable_start + sz;
> +
> +	return 0;
> +}
> +
> +static int __init snp_rmptable_init(void)
> +{
> +	if (!boot_cpu_has(X86_FEATURE_SEV_SNP))

cpu_feature_enabled

> +		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", __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 before enabling
> + * the SNP feature we need to ensure that IOMMU supports the SEV-SNP feature.
> + * The iommu_sev_snp_support() is used for checking the feature, and 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.

Thx.

-- 
Regards/Gruss,
    Boris.

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