| Message ID | 20240309010929.1403984-5-seanjc@google.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | KVM: VMX: Drop MTRR virtualization, honor guest PAT | expand |
On 3/9/24 02:09, Sean Christopherson wrote: > From: Yan Zhao <yan.y.zhao@intel.com> > > Ensure a full memory barrier is emitted in the VM-Exit path, as a full > barrier is required on Intel CPUs to evict WC buffers. This will allow > unconditionally honoring guest PAT on Intel CPUs that support self-snoop. > > As srcu_read_lock() is always called in the VM-Exit path and it internally > has a smp_mb(), call smp_mb__after_srcu_read_lock() to avoid adding a > second fence and make sure smp_mb() is called without dependency on > implementation details of srcu_read_lock(). Do you really need mfence or is a locked operation enough? mfence is mb(), not smp_mb(). Paolo > + /* > + * Call this to ensure WC buffers in guest are evicted after each VM > + * Exit, so that the evicted WC writes can be snooped across all cpus > + */ > + smp_mb__after_srcu_read_lock();
On Fri, Jun 21, 2024 at 12:38:21AM +0200, Paolo Bonzini wrote: > On 3/9/24 02:09, Sean Christopherson wrote: > > From: Yan Zhao <yan.y.zhao@intel.com> > > > > Ensure a full memory barrier is emitted in the VM-Exit path, as a full > > barrier is required on Intel CPUs to evict WC buffers. This will allow > > unconditionally honoring guest PAT on Intel CPUs that support self-snoop. > > > > As srcu_read_lock() is always called in the VM-Exit path and it internally > > has a smp_mb(), call smp_mb__after_srcu_read_lock() to avoid adding a > > second fence and make sure smp_mb() is called without dependency on > > implementation details of srcu_read_lock(). > > Do you really need mfence or is a locked operation enough? mfence is mb(), > not smp_mb(). We only need smp_mb(), which is supplied by the srcu_read_lock() function. For now, anyway. If we ever figure out how to get by with lighter-weight ordering for srcu_read_lock(), then we will add an smp_mb() to smp_mb__after_srcu_read_lock() to compensate. Thanx, Paul > Paolo > > > + /* > > + * Call this to ensure WC buffers in guest are evicted after each VM > > + * Exit, so that the evicted WC writes can be snooped across all cpus > > + */ > > + smp_mb__after_srcu_read_lock(); >
On Fri, Jun 21, 2024 at 12:38:21AM +0200, Paolo Bonzini wrote: > On 3/9/24 02:09, Sean Christopherson wrote: > > From: Yan Zhao <yan.y.zhao@intel.com> > > > > Ensure a full memory barrier is emitted in the VM-Exit path, as a full > > barrier is required on Intel CPUs to evict WC buffers. This will allow > > unconditionally honoring guest PAT on Intel CPUs that support self-snoop. > > > > As srcu_read_lock() is always called in the VM-Exit path and it internally > > has a smp_mb(), call smp_mb__after_srcu_read_lock() to avoid adding a > > second fence and make sure smp_mb() is called without dependency on > > implementation details of srcu_read_lock(). > > Do you really need mfence or is a locked operation enough? mfence is mb(), > not smp_mb(). > A locked operation should be enough, since the barrier here is to evict partially filled WC buffers. " If the WC buffer is partially filled, the writes may be delayed until the next occurrence of a serializing event; such as an SFENCE or MFENCE instruction, CPUID or other serializing instruction, a read or write to uncached memory, an interrupt occurrence, or an execution of a LOCK instruction (including one with an XACQUIRE or XRELEASE prefix). " > > > + /* > > + * Call this to ensure WC buffers in guest are evicted after each VM > > + * Exit, so that the evicted WC writes can be snooped across all cpus > > + */ > > + smp_mb__after_srcu_read_lock(); >
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index 276ae56dd888..69e815df1699 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -11082,6 +11082,12 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu) kvm_vcpu_srcu_read_lock(vcpu); + /* + * Call this to ensure WC buffers in guest are evicted after each VM + * Exit, so that the evicted WC writes can be snooped across all cpus + */ + smp_mb__after_srcu_read_lock(); + /* * Profile KVM exit RIPs: */