Re: [Bug 103321] New: NPT page attribute support causes extreme slowdown

[Paolo Bonzini <pbonzini@redhat.com>]

On 02/09/2015 21:01, Sebastian Schütte wrote:
> I inserted some printk() lines into init_vmcb() around the call of
> svm_set_guest_pat() to print out the g_pat value as well as
> svm->vcpu.vcpu_id and noticed that something was off:
> 
> Initially, the PATs of all VCPUs are set to 0x0606060606060606.
> However, after attaching some devices (vfio-pci enabling device and
> vfio_ecap_init lines are being printed) init_vmcb() is only called
> again for vcpu_id > 0. Unless g_pat is changed somewhere else, VCPU
> #0 remains set to 0x0606060606060606 (according to comments in
> svm_set_guest_pat() this is bad for assigned devices) while all other
> VCPUs use 0x0007040600070406.
> 
> I'd guess that could explain the slowdown.
> 

Hi, sorry for the delay.  If you change KVM like this

diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
index 27f57fc05bc7..3ce878c5fde8 100644
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -1155,10 +1155,7 @@ static void svm_set_guest_pat(struct vcpu_svm *svm, u64 *g_pat)
 	 * have assigned devices, however, we cannot force WB for RAM
 	 * pages only, so use the guest PAT directly.
 	 */
-	if (!kvm_arch_has_assigned_device(vcpu->kvm))
-		*g_pat = 0x0606060606060606;
-	else
-		*g_pat = vcpu->arch.pat;
+	*g_pat = vcpu->arch.pat;
 }
 
 static u64 svm_get_mt_mask(struct kvm_vcpu *vcpu, gfn_t gfn, bool is_mmio)

(obviously not a valid patch for upstream patch, but good enough for
testing) do you get normal speed?

Paolo