Re: [PATCH v10 4/6] x86/sev: Add support to perform RMP optimizations asynchronously

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] Re-initializing SNP after a partial shutdown bypasses the RMPOPT optimization pass, leaving the new RMP table unoptimized.
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commit 2d85f8f029da9849b60d0154807adccc997f6502
Author: Ashish Kalra <ashish.kalra@amd.com>

x86/sev: Add support to perform RMP optimizations asynchronously

This patch introduces support for the RMPOPT instruction to minimize performance
overhead by skipping RMP checks on 1GB memory ranges known not to contain
SEV-SNP guest memory. It adds a dedicated workqueue to perform these
optimizations asynchronously.

> diff --git a/arch/x86/virt/svm/sev.c b/arch/x86/virt/svm/sev.c
> index 04a58ac4339cb..40b06e959ee87 100644
> --- a/arch/x86/virt/svm/sev.c
> +++ b/arch/x86/virt/svm/sev.c

[ ... ]

> @@ -625,14 +743,42 @@ void snp_setup_rmpopt(void)
>  	if (!cpu_feature_enabled(X86_FEATURE_RMPOPT) || !rmpopt_capable)
>  		return;
>  
> +	guard(mutex)(&rmpopt_wq_mutex);
> +
> +	/*
> +	 * Guard against re-initialization.  When SNP_SHUTDOWN_EX is issued
> +	 * with x86_snp_shutdown=0, snp_shutdown() is not called and
> +	 * rmpopt_cleanup() is skipped, but snp_initialized is still cleared.
> +	 * A subsequent __sev_snp_init_locked() would call snp_setup_rmpopt()
> +	 * again, leaking the existing workqueue, delayed work, and cpumask
> +	 * state.
> +	 */
> +	if (rmpopt_wq)
> +		return;

[Severity: Medium]
Will this early return prevent the initial RMPOPT optimization pass if SNP is
re-initialized after a partial shutdown?

If a partial shutdown occurs where rmpopt_cleanup() is skipped, rmpopt_wq
remains allocated. When snp_setup_rmpopt() is called again during
re-initialization, returning early here skips the queue_delayed_work()
call at the end of the function.

Does this mean the system will run without RMP optimizations on the newly
initialized RMP table, incurring RMP check overhead until a later event
triggers a rescan?

[ ... ]

> +	/* Limit memory scanning to 2TB of RAM */
> +	if ((rmpopt_pa_end - rmpopt_pa_start) > SZ_2T) {
> +		pr_info("RMPOPT coverage limited to 2TB; memory above 0x%llx not optimized\n",
> +			rmpopt_pa_start + SZ_2T);
> +		rmpopt_pa_end = rmpopt_pa_start + SZ_2T;
> +	}
> +
> +	/*
> +	 * Once all per-CPU RMPOPT tables have been configured, enable RMPOPT
> +	 * optimizations on all physical memory.
> +	 */
> +	queue_delayed_work(rmpopt_wq, &rmpopt_delayed_work, 0);
>  }

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Sashiko AI review · https://sashiko.dev/#/patchset/cover.1782841284.git.ashish.kalra@amd.com?part=4