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Sashiko AI review found 2 potential issue(s) to consider: > - [High] Race condition between KVM VM destruction and CCP unbind allows RMPOPT work to be queued after SNP shutdown, leading to a potential kernel panic. > - [High] System lockup due to broadcasting a cold-cache memory scan to all CPUs with interrupts disabled. > -- > > commit 5610134a51dd4a088684e2eb5968c3e869f5727c > Author: Ashish Kalra > > x86/sev: Re-enable RMP optimizations on SNP guest shutdown > > This commit re-enables RMP optimizations on SNP guest shutdown by scheduling > a delayed work pass. It introduces snp_rmpopt_all_physmem() to queue the scan > 10 seconds after a guest termination. The delayed work batches multiple guest > teardowns into a single optimization pass. > >> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c >> index 5705723f1f412..d8e6b8a08b79f 100644 >> --- a/arch/x86/kvm/svm/sev.c >> +++ b/arch/x86/kvm/svm/sev.c > [ ... ] >> @@ -3032,6 +3032,8 @@ void sev_vm_destroy(struct kvm *kvm) >> */ >> if (snp_decommission_context(kvm)) >> return; >> + >> + snp_rmpopt_all_physmem(); > > [Severity: High] > Does this create a race condition with CCP unbind that can lead to a host panic? > > If the CCP device is unbound or shut down, snp_shutdown() is invoked, which > disables RMPOPT by cancelling pending work via rmpopt_disable(). However, > rmpopt_disable() does not set rmpopt_wq = NULL: > > arch/x86/virt/svm/sev.c:rmpopt_disable() { > guard(mutex)(&rmpopt_wq_mutex); > > if (rmpopt_wq) > cancel_delayed_work_sync(&rmpopt_delayed_work); > } > > If KVM concurrently destroys a VM, it calls snp_rmpopt_all_physmem() here. > Because rmpopt_wq is still valid, the background scan is blindly re-queued. > > When the delayed work executes, it will issue the RMPOPT instruction while > SNP is disabled in hardware (SYSCFG_SNP_EN == 0), causing an Invalid Opcode > (#UD) exception and panicking the host system. The concern is valid in principle. In practice, though, it requires an unsupported configuration: - A full SNP shutdown (SNP_SHUTDOWN, which clears SnpEn) only succeeds when there are no active SNP guests. The only way snp_rmpopt_all_physmem() can re‑queue the pass after SnpEn has been cleared is if the CCP/SEV platform is being torn down (driver unbind/removal or system shutdown) concurrently with an in‑flight SNP guest teardown. - Tearing down the SEV platform while SNP guests are still being destroyed is not a supported operation. Given that, we keep the RMPOPT setup/teardown minimal — set up once, and re‑queue the pass only on re‑initialization — rather than add persistent "enabled" state solely to guard an unsupported teardown ordering. Thanks, Ashish > >> } else { >> sev_unbind_asid(kvm, sev->handle); >> } >> diff --git a/arch/x86/virt/svm/sev.c b/arch/x86/virt/svm/sev.c >> index 35678b1f535d4..c16f826423903 100644 >> --- a/arch/x86/virt/svm/sev.c >> +++ b/arch/x86/virt/svm/sev.c >> @@ -640,6 +640,37 @@ static void do_rmpopt_work(struct work_struct *work) >> on_each_cpu_mask(cpu_primary_thread_mask, rmpopt_scan_range, NULL, true); > > [Severity: High] > Can this cause a system lockup by broadcasting a cold-cache memory scan to all > CPUs with interrupts disabled? > > When an SNP guest shuts down, it queues rmpopt_delayed_work. The worker > function do_rmpopt_work() attempts to warm up the cache first: > > arch/x86/virt/svm/sev.c:do_rmpopt_work() { > rmpopt_scan_range(NULL); > on_each_cpu_mask(cpu_primary_thread_mask, rmpopt_scan_range, NULL, true); > } > > Because the RMPOPT hardware cache is per-CPU, running rmpopt_scan_range() > locally only warms up the cache for the worker's CPU. The subsequent > on_each_cpu_mask() forces all other CPUs to run the scan with interrupts > disabled. > > Since all other CPUs have a cold cache, they perform a massive memory scan > (up to 2TB) while IRQs are disabled, which can cause immense latency and > lead to host lockup (exceeding NMI watchdog or RCU stall timeouts). > The RMPOPT optimization results are held in shared memory, not per‑CPU caches, so after the local warm‑up the follower CPUs read the already‑computed results from shared memory and short‑circuit. Measured via follower_timings, the worst‑case follower IPI callback is ≈1.3 ms (1,341,505 ns) — ~4 orders of magnitude under the soft‑lockup/NMI/RCU‑stall thresholds. Thanks, Ashish >> } >> >> +/* >> + * Delay, in milliseconds, before the RMP re-optimization pass runs after an SNP > [ ... ] >