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>> } >> >> +/* >> + * Delay, in milliseconds, before the RMP re-optimization pass runs after an >> + * SNP guest is torn down. > [ ... ] >> +void snp_rmpopt_all_physmem(void) >> +{ >> + if (!rmpopt_capable()) >> + return; >> + >> + guard(mutex)(&rmpopt_wq_mutex); >> + >> + if (!rmpopt_wq) >> + return; >> + >> + mod_delayed_work(rmpopt_wq, &rmpopt_delayed_work, >> + msecs_to_jiffies(RMPOPT_WORK_TIMEOUT)); >> +} > [Severity: High] > This is a pre-existing issue, but queueing the do_rmpopt_work task upon guest > teardown exposes the host to severe CPU stalls. > > Looking at do_rmpopt_work, it broadcasts an IPI to all primary threads: > > arch/x86/virt/svm/sev.c:do_rmpopt_work() { > ... > migrate_disable(); > rmpopt_scan_range(NULL); > migrate_enable(); > > on_each_cpu_mask(cpu_primary_thread_mask, rmpopt_scan_range, NULL, true); > } > > The IPI handler rmpopt_scan_range executes the rmpopt hardware instruction > in hardirq context over up to 2TB of memory: > > arch/x86/virt/svm/sev.c:rmpopt_scan_range() { > ... > for (pa = rmpopt_pa_start; pa < rmpopt_pa_end; pa += SZ_1G) > rmpopt(pa); > ... > } > > Since rmpopt is a very slow instruction, does iterating it over massive > memory ranges inside an IPI handler lead to soft lockups or host denial of > service, especially if frequent VM churn repeatedly tears down SNP guests > to trigger this scan? Two things bound this: 1. Coalescing. snp_rmpopt_all_physmem() uses mod_delayed_work() with a 10 s delay that is reset on every call, so a burst of guest teardowns does not queue one scan per teardown — the pass runs once, 10 s after the last teardown. Frequent VM churn is coalesced into a single pass, not amplified. 2. The IPI does not run an uncached scan. do_rmpopt_work() first runs rmpopt_scan_range() once on the worker's own CPU under migrate_disable() (interrupts on, preemptible), which does the expensive part and populates the shared RMPOPT table — all primary threads share the same RMPOPT_BASE. Only then does on_each_cpu_mask() fan out, and those RMPOPTs are cache hits (reads of the already-computed result), so the IRQ-disabled window per CPU is bounded by the cache-hit cost, not the scan cost — measured at ~320 ms over the largest configuration tested, well under the soft-lockup/NMI thresholds. > > Additionally, because rmpopt_wq is created as WQ_UNBOUND, the worker > thread might run on an SMT sibling. If so, wouldn't the initial local scan > execute on a CPU where MSR_AMD64_RMPOPT_BASE was never configured, > potentially leading to undefined behavior or faults? MSR_AMD64_RMPOPT_BASE has core scope, so programming it on each core's primary thread configures it for both SMT threads of that core. A WQ_UNBOUND worker that happens to run on a secondary thread therefore still executes RMPOPT against a validly-configured RMPOPT_BASE (its core's), and since CPU hotplug is disabled while SNP is active, every core stays configured. There is no CPU on which RMPOPT_BASE is unconfigured, so no fault or undefined behaviour. Thanks, Ashish