Re: [PATCH v2 2/7] x86/sev: add support for enabling RMPOPT

[Dave Hansen]

On 3/2/26 13:35, Ashish Kalra wrote:
> The new RMPOPT instruction sets bits in a per-CPU RMPOPT table, which
> indicates whether specific 1GB physical memory regions contain SEV-SNP
> guest memory.

Honestly, this is an implementation detail that we don't need to know
about in the kernel. It's also not even factually correct. The
instruction _might_ not set any bits, either because there is SEV-SNP
memory or because it's being run in query mode.

	The new RMPOPT instruction helps manage per-CPU RMP optimization
	structures inside the CPU. It takes a 1GB-aligned physical
	address and either returns the status of the optimizations or
	tries to enable the optimizations.

> Per-CPU RMPOPT tables support at most 2 TB of addressable memory for
> RMP optimizations.
> 
> Initialize the per-CPU RMPOPT table base to the starting physical
> address. This enables RMP optimization for up to 2 TB of system RAM on
> all CPUs.

The reset looks good.

> diff --git a/arch/x86/include/asm/msr-index.h b/arch/x86/include/asm/msr-index.h
> index da5275d8eda6..8e7da03abd5b 100644
> --- a/arch/x86/include/asm/msr-index.h
> +++ b/arch/x86/include/asm/msr-index.h
> @@ -753,6 +753,9 @@
>  #define MSR_AMD64_SEG_RMP_ENABLED_BIT	0
>  #define MSR_AMD64_SEG_RMP_ENABLED	BIT_ULL(MSR_AMD64_SEG_RMP_ENABLED_BIT)
>  #define MSR_AMD64_RMP_SEGMENT_SHIFT(x)	(((x) & GENMASK_ULL(13, 8)) >> 8)
> +#define MSR_AMD64_RMPOPT_BASE		0xc0010139
> +#define MSR_AMD64_RMPOPT_ENABLE_BIT	0
> +#define MSR_AMD64_RMPOPT_ENABLE		BIT_ULL(MSR_AMD64_RMPOPT_ENABLE_BIT)
>  
>  #define MSR_SVSM_CAA			0xc001f000
>  
> diff --git a/arch/x86/virt/svm/sev.c b/arch/x86/virt/svm/sev.c
> index a4f3a364fb65..405199c2f563 100644
> --- a/arch/x86/virt/svm/sev.c
> +++ b/arch/x86/virt/svm/sev.c
> @@ -500,6 +500,41 @@ static bool __init setup_rmptable(void)
>  	}
>  }
>  
> +static void __configure_rmpopt(void *val)
> +{
> +	u64 rmpopt_base = ((u64)val & PUD_MASK) | MSR_AMD64_RMPOPT_ENABLE;
> +
> +	wrmsrq(MSR_AMD64_RMPOPT_BASE, rmpopt_base);
> +}
> +
> +static __init void configure_and_enable_rmpopt(void)
> +{
> +	phys_addr_t pa_start = ALIGN_DOWN(PFN_PHYS(min_low_pfn), PUD_SIZE);
> +
> +	if (!cpu_feature_enabled(X86_FEATURE_RMPOPT)) {
> +		pr_debug("RMPOPT not supported on this platform\n");
> +		return;
> +	}
> +
> +	if (!cc_platform_has(CC_ATTR_HOST_SEV_SNP)) {
> +		pr_debug("RMPOPT optimizations not enabled as SNP support is not enabled\n");
> +		return;
> +	}

To be honest, I think those two are just plain noise ^^.

> +	if (!(rmp_cfg & MSR_AMD64_SEG_RMP_ENABLED)) {
> +		pr_info("RMPOPT optimizations not enabled, segmented RMP required\n");
> +		return;
> +	}
> +
> +	/*
> +	 * Per-CPU RMPOPT tables support at most 2 TB of addressable memory for RMP optimizations.
> +	 *
> +	 * Set per-core RMPOPT base to min_low_pfn to enable RMP optimization for
> +	 * up to 2TB of system RAM on all CPUs.
> +	 */

Please at least be consistent with your comments. This is both over 80
columns *and* not even consistent in the two sentences.

> +	on_each_cpu_mask(cpu_online_mask, __configure_rmpopt, (void *)pa_start, true);
> +}

What's wrong with:

	u64 rmpopt_base = pa_start | MSR_AMD64_RMPOPT_ENABLE;
	...
	for_each_online_cpu(cpu)
		wrmsrq_on_cpu(cpu, MSR_AMD64_RMPOPT_BASE, rmpopt_base);

Then there's at least no ugly casting.