On Tue, Mar 05, 2024, Zheyun Shen wrote: > On Mon, Mar 04, 2024, Sean Christopherson wrote: > > Instead of copy+paste WBINVD+cpumask_clear() everywhere, add a prep patch to > > replace relevant open coded calls to wbinvd_on_all_cpus() with calls to > > sev_guest_memory_reclaimed(). Then only sev_guest_memory_reclaimed() needs to > > updated, and IMO it helps document why KVM is blasting WBINVD. > > > I'm also pretty sure this should be a cpumask_var_t, and dynamically allocated > > as appropriate. And at that point, it should be allocated and filled if and only > > if the CPU doesn't have X86_FEATURE_SME_COHERENT > > I notice that several callers of wbinvd_on_all_cpus() must use wbinvd to flush cache > instead of using clflush or just doing nothing if the CPU has X86_FEATURE_SME_COHERENT, > according to https://github.com/AMDESE/linux/commit/2e2409afe5f0c284c7dfe5504058e8d115806a7d > Therefore, I think the flush operation should be divided into two functions. One is the > optimized wbinvd, which does not consider X86_FEATURE_SME_COHERENT, and the other is > sev_guest_memory_reclaimed(), which should use clflush instead of wbinvd in case of > X86_FEATURE_SME_COHERENT. Thus the cpumask struct should be exist whether the CPU has > X86_FEATURE_SME_COHERENT or not. FWIW, the usage of sev_flush_asids() isn't tied to a single VM, i.e. KVM can't use per-VM tracking in that case. But... > Besides, if we consider X86_FEATURE_SME_COHERENT to get rid of wbinvd in sev_guest_memory_reclaimed(), > we should ensure the clflush is called on corresponding addresses, as mentioned in > https://github.com/AMDESE/linux/commit/d45829b351ee6ec5f54dd55e6aca1f44fe239fe6 > However, caller of sev_guest_memory_reclaimed() (e.g., kvm_mmu_notifier_invalidate_range_start()) > only get HVA belongs to userspace(e.g., qemu), so calling clflush with this HVA may > lead to a page fault in kernel. I was wondering if notifying userspace applications to > do clflush themselves is the only solution here. But for the sake of safety, maybe KVM > cannot left the work for untrusted userspace applications? Ugh, right, I forgot the whole mess with userspace virtual addresses. Bummer. > Or should I just temporarily ignore the X86_FEATURE_SME_COHERENT scenario > which is hard to implement, and just refine the patch only for > wbinvd_on_all_cpus() ? Ignore X86_FEATURE_SME_COHERENT and just refine the patch to optimize WBINVDs that are tied to a specific VM. I simply forgot that KVM only uses CLFLUSHOPT when purging VMSA pages.
