Re: [RFC PATCH v2 09/25] KVM: SVM: Use a static ASID per vCPU

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] KVM fails to load on systems where all guest ASIDs are assigned to SEV (`nr_asids == max_sev_asid + 1`), breaking SEV-only environments.
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commit 6e1ef9a91f738606e7d8f9e36c1659d383c52061
Author: Yosry Ahmed <yosry@kernel.org>

KVM: SVM: Use a static ASID per vCPU

This patch switches KVM SVM from dynamic ASID allocation to a static
per-vCPU ASID to avoid full TLB flushes and simplify the code. It introduces
a fallback ASID logic and relies on TLB tag tracking for the pool of
available ASIDs.

> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index 5d4c45d788b54..fae5cb7102010 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c

[ ... ]

> @@ -5751,6 +5744,17 @@ static __init int svm_hardware_setup(void)
>  
>  	kvm_caps.inapplicable_quirks &= ~KVM_X86_QUIRK_CD_NW_CLEARED;
>  
> +	/* Consumes max_sev_asid initialized by sev_hardware_setup() */
> +	min_asid = max_sev_asid + 1;
> +	nr_asids = cpuid_ebx(SVM_CPUID_FUNC);
> +	r = kvm_init_tlb_tags(min_asid, nr_asids - min_asid);

[Severity: High]
Will this cause KVM to fail loading on systems where the BIOS configures all
available guest ASIDs for SEV?

If max_sev_asid equals nr_asids - 1 (a common configuration for cloud
servers dedicated to Confidential VMs), then min_asid will equal nr_asids.
This makes nr_asids - min_asid evaluate to 0.

When kvm_init_tlb_tags() is called with a count of 0, it returns -EINVAL,
which causes svm_hardware_setup() to fail and prevents the KVM module from
loading entirely. Prior to this patch, KVM would successfully load in this
configuration and allow SEV guests to run normally.

> +	if (r)
> +		goto err;
> +
> +	fallback_asid = kvm_alloc_tlb_tag();
> +	if (!fallback_asid)
> +		goto err;
> +
>  	for_each_possible_cpu(cpu) {
>  		r = svm_cpu_init(cpu);
>  		if (r)

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Sashiko AI review · https://sashiko.dev/#/patchset/20260616004155.1435766-1-yosry@kernel.org?part=9