Re: [PATCH v6 2/2] KVM: SEV: Prefer WBNOINVD over WBINVD for cache maintenance efficiency

Tom Lendacky
2025-02-04T17:00:31Z

On 1/31/25 18:02, Kevin Loughlin wrote:
> AMD CPUs currently execute WBINVD in the host when unregistering SEV
> guest memory or when deactivating SEV guests. Such cache maintenance is
> performed to prevent data corruption, wherein the encrypted (C=1)
> version of a dirty cache line might otherwise only be written back
> after the memory is written in a different context (ex: C=0), yielding
> corruption. However, WBINVD is performance-costly, especially because
> it invalidates processor caches.
>
> Strictly-speaking, unless the SEV ASID is being recycled (meaning the
> SNP firmware requires the use of WBINVD prior to DF_FLUSH), the cache
> invalidation triggered by WBINVD is unnecessary; only the writeback is
> needed to prevent data corruption in remaining scenarios.
>
> To improve performance in these scenarios, use WBNOINVD when available
> instead of WBINVD. WBNOINVD still writes back all dirty lines
> (preventing host data corruption by SEV guests) but does *not*
> invalidate processor caches. Note that the implementation of wbnoinvd()
> ensures fall back to WBINVD if WBNOINVD is unavailable.
>
> In anticipation of forthcoming optimizations to limit the WBNOINVD only
> to physical CPUs that have executed SEV guests, place the call to
> wbnoinvd_on_all_cpus() in a wrapper function sev_writeback_caches().
>
> Signed-off-by: Kevin Loughlin <kevinloughlin@google.com>
> Reviewed-by: Mingwei Zhang <mizhang@google.com>

Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>

> ---
> arch/x86/kvm/svm/sev.c | 41 +++++++++++++++++++++--------------------
> 1 file changed, 21 insertions(+), 20 deletions(-)
>
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index fe6cc763fd51..f10f1c53345e 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -116,6 +116,7 @@ static int sev_flush_asids(unsigned int min_asid, unsigned int max_asid)
> */
> down_write(&sev_deactivate_lock);
>
> + /* SNP firmware requires use of WBINVD for ASID recycling. */
> wbinvd_on_all_cpus();
>
> if (sev_snp_enabled)
> @@ -710,6 +711,16 @@ static void sev_clflush_pages(struct page *pages[], unsigned long npages)
> }
> }
>
> +static inline void sev_writeback_caches(void)
> +{
> + /*
> + * Ensure that all dirty guest tagged cache entries are written back
> + * before releasing the pages back to the system for use. CLFLUSH will
> + * not do this without SME_COHERENT, so issue a WBNOINVD.
> + */
> + wbnoinvd_on_all_cpus();
> +}
> +
> static unsigned long get_num_contig_pages(unsigned long idx,
> struct page **inpages, unsigned long npages)
> {
> @@ -2773,12 +2784,7 @@ int sev_mem_enc_unregister_region(struct kvm *kvm,
> goto failed;
> }
>
> - /*
> - * Ensure that all guest tagged cache entries are flushed before
> - * releasing the pages back to the system for use. CLFLUSH will
> - * not do this, so issue a WBINVD.
> - */
> - wbinvd_on_all_cpus();
> + sev_writeback_caches();
>
> __unregister_enc_region_locked(kvm, region);
>
> @@ -2899,12 +2905,7 @@ void sev_vm_destroy(struct kvm *kvm)
> return;
> }
>
> - /*
> - * Ensure that all guest tagged cache entries are flushed before
> - * releasing the pages back to the system for use. CLFLUSH will
> - * not do this, so issue a WBINVD.
> - */
> - wbinvd_on_all_cpus();
> + sev_writeback_caches();
>
> /*
> * if userspace was terminated before unregistering the memory regions
> @@ -3126,16 +3127,16 @@ static void sev_flush_encrypted_page(struct kvm_vcpu *vcpu, void *va)
>
> /*
> * VM Page Flush takes a host virtual address and a guest ASID. Fall
> - * back to WBINVD if this faults so as not to make any problems worse
> + * back to WBNOINVD if this faults so as not to make any problems worse
> * by leaving stale encrypted data in the cache.
> */
> if (WARN_ON_ONCE(wrmsrl_safe(MSR_AMD64_VM_PAGE_FLUSH, addr | asid)))
> - goto do_wbinvd;
> + goto do_sev_writeback_caches;
>
> return;
>
> -do_wbinvd:
> - wbinvd_on_all_cpus();
> +do_sev_writeback_caches:
> + sev_writeback_caches();
> }
>
> void sev_guest_memory_reclaimed(struct kvm *kvm)
> @@ -3144,12 +3145,12 @@ void sev_guest_memory_reclaimed(struct kvm *kvm)
> * With SNP+gmem, private/encrypted memory is unreachable via the
> * hva-based mmu notifiers, so these events are only actually
> * pertaining to shared pages where there is no need to perform
> - * the WBINVD to flush associated caches.
> + * the WBNOINVD to flush associated caches.
> */
> if (!sev_guest(kvm) || sev_snp_guest(kvm))
> return;
>
> - wbinvd_on_all_cpus();
> + sev_writeback_caches();
> }
>
> void sev_free_vcpu(struct kvm_vcpu *vcpu)
> @@ -3858,7 +3859,7 @@ static int __sev_snp_update_protected_guest_state(struct kvm_vcpu *vcpu)
> * guest-mapped page rather than the initial one allocated
> * by KVM in svm->sev_es.vmsa. In theory, svm->sev_es.vmsa
> * could be free'd and cleaned up here, but that involves
> - * cleanups like wbinvd_on_all_cpus() which would ideally
> + * cleanups like sev_writeback_caches() which would ideally
> * be handled during teardown rather than guest boot.
> * Deferring that also allows the existing logic for SEV-ES
> * VMSAs to be re-used with minimal SNP-specific changes.
> @@ -4910,7 +4911,7 @@ void sev_gmem_invalidate(kvm_pfn_t start, kvm_pfn_t end)
>
> /*
> * SEV-ES avoids host/guest cache coherency issues through
> - * WBINVD hooks issued via MMU notifiers during run-time, and
> + * WBNOINVD hooks issued via MMU notifiers during run-time, and
> * KVM's VM destroy path at shutdown. Those MMU notifier events
> * don't cover gmem since there is no requirement to map pages
> * to a HVA in order to use them for a running guest. While the