Re: [PATCH v10 6/6] KVM: SEV: Perform RMP optimizations on SNP guest shutdown

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] Indefinite deferral of RMP optimizations via mod_delayed_work timer reset.
--

commit 5a78324bed688cb2c4b58d363233a323513bd9b4
Author: Ashish Kalra <ashish.kalra@amd.com>

KVM: SEV: Perform RMP optimizations on SNP guest shutdown

This commit performs RMP optimizations on memory freed by terminating SNP
guests. The scan is deferred so that a single optimization pass can batch
memory from multiple guests terminating in a short period of time.

> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index 4272293478766..c574849b15878 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
[ ... ]
> @@ -2992,6 +2992,16 @@ void sev_vm_destroy(struct kvm *kvm)
>  		 */
>  		if (snp_decommission_context(kvm))
>  			return;
> +
> +		/*
> +		 * Perform RMP optimizations on memory freed by terminating
> +		 * guests.  The scan is deferred, so it normally runs after
> +		 * sev_gmem_invalidate() has converted this guest's pages back to
> +		 * shared, and picks them up then.  A very large guest whose
> +		 * conversion has not finished by then is picked up by a later
> +		 * teardown's scan.
> +		 */
> +		snp_rmpopt_all_physmem();

[Severity: High]
Can an unprivileged user indefinitely defer the RMP optimizations by
repeatedly creating and destroying minimal SNP VMs?

Looking at snp_rmpopt_all_physmem(), it uses mod_delayed_work(), which
cancels any pending timer and pushes the execution timeout back on every
call.

If VMs are destroyed at a frequency higher than the timeout,
the background optimization task might be starved and never execute,
leading to degraded memory access performance.

Would using queue_delayed_work() instead prevent this starvation by
ensuring the timer is not pushed back if the work is already queued?

>  	} else {
>  		sev_unbind_asid(kvm, sev->handle);
>  	}

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/cover.1782841284.git.ashish.kalra@amd.com?part=6