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Date: Fri, 12 Jan 2024 15:49:31 +0100
From: Borislav Petkov <bp@alien8.de>
To: Michael Roth <michael.roth@amd.com>
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	Brijesh Singh <brijesh.singh@amd.com>
Subject: Re: [PATCH v1 10/26] x86/sev: Add helper functions for RMPUPDATE and
 PSMASH instruction
Message-ID: <20240112144931.GCZaFRewg1ft-oS_rY@fat_crate.local>
References: <20231230161954.569267-1-michael.roth@amd.com>
 <20231230161954.569267-11-michael.roth@amd.com>
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In-Reply-To: <20231230161954.569267-11-michael.roth@amd.com>

On Sat, Dec 30, 2023 at 10:19:38AM -0600, Michael Roth wrote:
> +static int rmpupdate(u64 pfn, struct rmp_state *state)
> +{
> +	unsigned long paddr = pfn << PAGE_SHIFT;
> +	int ret;
> +
> +	if (!cpu_feature_enabled(X86_FEATURE_SEV_SNP))
> +		return -ENODEV;
> +

Let's document this deal here and leave the door open for potential
future improvements:

---

diff --git a/arch/x86/virt/svm/sev.c b/arch/x86/virt/svm/sev.c
index c9156ce47c77..1fbf9843c163 100644
--- a/arch/x86/virt/svm/sev.c
+++ b/arch/x86/virt/svm/sev.c
@@ -374,6 +374,21 @@ static int rmpupdate(u64 pfn, struct rmp_state *state)
 	if (!cpu_feature_enabled(X86_FEATURE_SEV_SNP))
 		return -ENODEV;
 
+	/*
+	 * It is expected that those operations are seldom enough so
+	 * that no mutual exclusion of updaters is needed and thus the
+	 * overlap error condition below should happen very seldomly and
+	 * would get resolved relatively quickly by the firmware.
+	 *
+	 * If not, one could consider introducing a mutex or so here to
+	 * sync concurrent RMP updates and thus diminish the amount of
+	 * cases where firmware needs to lock 2M ranges to protect
+	 * against concurrent updates.
+	 *
+	 * The optimal solution would be range locking to avoid locking
+	 * disjoint regions unnecessarily but there's no support for
+	 * that yet.
+	 */
 	do {
 		/* Binutils version 2.36 supports the RMPUPDATE mnemonic. */
 		asm volatile(".byte 0xF2, 0x0F, 0x01, 0xFE"

> +	do {
> +		/* Binutils version 2.36 supports the RMPUPDATE mnemonic. */
> +		asm volatile(".byte 0xF2, 0x0F, 0x01, 0xFE"
> +			     : "=a" (ret)
> +			     : "a" (paddr), "c" ((unsigned long)state)
> +			     : "memory", "cc");
> +	} while (ret == RMPUPDATE_FAIL_OVERLAP);
> +
> +	if (ret) {
> +		pr_err("RMPUPDATE failed for PFN %llx, ret: %d\n", pfn, ret);
> +		dump_rmpentry(pfn);
> +		dump_stack();
> +		return -EFAULT;
> +	}
> +
> +	return 0;
> +}


-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette