Re: [PATCH] KVM: SVM: Bump asid_generation on CPU online to avoid ASID collision after hotplug

[Paolo Bonzini <pbonzini@redhat.com>]

On 7/15/26 08:35, Nikunj A Dadhania wrote:
> If a vCPU stays scheduled out (or blocked) while the last pCPU it ran
> on goes through a hotplug cycle (online->offline->online), and the vCPU
> then resumes execution on the same pCPU, then it is possible for it to
> run with an ASID that has now been assigned to a different vCPU,
> resulting in stale TLB translations being used.
> 
> svm_enable_virtualization_cpu() resets asid_generation to 1 and sets
> next_asid to max_asid + 1 on every CPU online event, including hotplug
> cycles.  Because next_asid starts beyond the pool boundary, the first
> call to new_asid() after an online event always wraps the pool,
> incrementing asid_generation to 2 and assigning ASIDs starting from
> min_asid.
> 
> Consider two vCPUs from different VMs, vCPU-A pinned to CPU-X holding
> asid_generation=2 and ASID=N from before the hotplug event:
> 
>    1. CPU-X goes offline and back online: asid_generation resets to 1,
>       next_asid = max_asid + 1.
> 
>    2. One or more vCPUs migrate to CPU-X and call new_asid(), wrapping
>       the pool and consuming ASIDs starting from min_asid.  Eventually
>       vCPU-B from a different VM is assigned asid_generation=2, ASID=N
>       — the same ASID that vCPU-A held before the hotplug.
> 
>    3. vCPU-A enters pre_svm_run() on CPU-X: current_vmcb->cpu is
>       unchanged so the migration branch is skipped.  Its saved
>       asid_generation=2 matches sd->asid_generation=2, so the generation
>       check silently passes and vCPU-A continues running with ASID=N —
>       the same ASID just freshly assigned to vCPU-B.
> 
> Both vCPUs from different VMs now run on CPU-X with the same ASID,
> causing them to share NPT TLB entries and producing stale translations.
> 
> The collision manifests as a KVM internal error (Suberror: 1, emulation
> failure).  The NPT page fault reports a faulting GPA far outside the
> VM's physical memory range — a sign of stale TLB translations being
> used.  KVM falls back to instruction emulation, which fails on
> FPU/XSave instructions (XRSTOR, STMXCSR) that the emulator does not
> implement.
> 
> Fix this by incrementing asid_generation instead of resetting it to 1
> in svm_enable_virtualization_cpu().  On module load, asid_generation
> starts at 0 (memset) and the increment produces 1, identical to the
> old behaviour.  On subsequent hotplug cycles the generation advances
> beyond any value a vCPU previously observed on this CPU, so the
> generation check in pre_svm_run() reliably forces new_asid() on every
> vCPU after every hotplug cycle.
> 
> Fixes: 774c47f1d78e ("[PATCH] KVM: cpu hotplug support")
> Reported-by: Chandrakanth Silveru <Chandrakanth.Silveru@amd.com>
> Tested-by: Srikanth Aithal <Srikanth.Aithal@amd.com>
> Reviewed-by: K Prateek Nayak <kprateek.nayak@amd.com>
> Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
> Signed-off-by: Nikunj A Dadhania <nikunj@amd.com>

Queued, thanks.

Paolo

> ---
>   arch/x86/kvm/svm/svm.c | 7 ++++++-
>   1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index 91286d46d13ad..c2c6487bd5bd1 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -571,7 +571,12 @@ static int svm_enable_virtualization_cpu(void)
>   		return r;
>   
>   	sd = per_cpu_ptr(&svm_data, me);
> -	sd->asid_generation = 1;
> +	/*
> +	 * Bump the current asid_generation value to ensure any vCPU that
> +	 * previously ran on this CPU sees a stale generation and is forced
> +	 * to acquire a new ASID, preventing a latent ASID collision.
> +	 */
> +	sd->asid_generation++;
>   	sd->max_asid = cpuid_ebx(SVM_CPUID_FUNC) - 1;
>   	sd->next_asid = sd->max_asid + 1;
>   	sd->min_asid = max_sev_asid + 1;