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Peter Anvin" , Paolo Bonzini , linux-kernel@vger.kernel.org, kvm@vger.kernel.org Content-Type: text/plain; charset="us-ascii" On Wed, Oct 15, 2025, Brendan Jackman wrote: > Currently the tracking of the need to flush L1D for L1TF is tracked by > two bits: one per-CPU and one per-vCPU. > > The per-vCPU bit is always set when the vCPU shows up on a core, so > there is no interesting state that's truly per-vCPU. Indeed, this is a > requirement, since L1D is a part of the physical CPU. > > So simplify this by combining the two bits. > > The vCPU bit was being written from preemption-enabled regions. For > those cases, use raw_cpu_write() (via a variant of the setter function) > to avoid DEBUG_PREEMPT failures. If the vCPU is getting migrated, the > CPU that gets its bit set in these paths is not important; vcpu_load() > must always set it on the destination CPU before the guest is resumed. > > Signed-off-by: Brendan Jackman > --- ... > @@ -78,6 +79,11 @@ static __always_inline void kvm_set_cpu_l1tf_flush_l1d(void) > __this_cpu_write(irq_stat.kvm_cpu_l1tf_flush_l1d, 1); > } > > +static __always_inline void kvm_set_cpu_l1tf_flush_l1d_raw(void) > +{ > + raw_cpu_write(irq_stat.kvm_cpu_l1tf_flush_l1d, 1); > +} TL;DR: I'll post a v3 with a slightly tweaked version of this patch at the end. Rather than add a "raw" variant, I would rather have a wrapper in arch/x86/kvm/x86.h that disables preemption, with a comment explaining why it's ok to enable preemption after setting the per-CPU flag. Without such a comment, choosing between the two variants looks entirely random Alternatively, all writes could be raw, but that feels wrong/weird, and in practice disabling preemption in the relevant paths is a complete non-issue. Gah, I followed a tangential thought about the "cost" of disabling/enabling preemtion and ended up with a 4-patch series. All of this code really should be conditioned on CONFIG_CPU_MITIGATIONS=y. With that, the wrapper can be: static __always_inline void kvm_request_l1tf_flush_l1d(void) { #if IS_ENABLED(CONFIG_CPU_MITIGATIONS) && IS_ENABLED(CONFIG_KVM_INTEL) /* * Temporarily disable preemption (if necessary) as the tracking is * per-CPU. If the current vCPU task is migrated to a different CPU * before the next VM-Entry, then kvm_arch_vcpu_load() will pend a * flush on the new CPU. */ guard(preempt)(); kvm_set_cpu_l1tf_flush_l1d(); #endif } and kvm_set_cpu_l1tf_flush_l1d() and irq_cpustat_t.kvm_cpu_l1tf_flush_l1d can likewise be gated on CONFIG_CPU_MITIGATIONS && CONFIG_KVM_INTEL. > + > static __always_inline void kvm_clear_cpu_l1tf_flush_l1d(void) > { > __this_cpu_write(irq_stat.kvm_cpu_l1tf_flush_l1d, 0); > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h > index 48598d017d6f3f07263a2ffffe670be2658eb9cb..fcdc65ab13d8383018577aacf19e832e6c4ceb0b 100644 > --- a/arch/x86/include/asm/kvm_host.h > +++ b/arch/x86/include/asm/kvm_host.h > @@ -1055,9 +1055,6 @@ struct kvm_vcpu_arch { > /* be preempted when it's in kernel-mode(cpl=0) */ > bool preempted_in_kernel; > > - /* Flush the L1 Data cache for L1TF mitigation on VMENTER */ > - bool l1tf_flush_l1d; > - > /* Host CPU on which VM-entry was most recently attempted */ > int last_vmentry_cpu; > > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c > index 667d66cf76d5e52c22f9517914307244ae868eea..8c0dce401a42d977756ca82d249bb33c858b9c9f 100644 > --- a/arch/x86/kvm/mmu/mmu.c > +++ b/arch/x86/kvm/mmu/mmu.c > @@ -4859,7 +4859,7 @@ int kvm_handle_page_fault(struct kvm_vcpu *vcpu, u64 error_code, > */ > BUILD_BUG_ON(lower_32_bits(PFERR_SYNTHETIC_MASK)); > > - vcpu->arch.l1tf_flush_l1d = true; > + kvm_set_cpu_l1tf_flush_l1d(); This is wrong, kvm_handle_page_fault() runs with preemption enabled.