On 01/06/2010 05:18 AM, Avi Kivity wrote:
+static void svm_fpu_deactivate(struct kvm_vcpu *vcpu)
+{
+ struct vcpu_svm *svm = to_svm(vcpu);
+
+ if (npt_enabled) {
+ /* hack: npt requires active fpu at this time */
+ vcpu->fpu_active = 1;
+ return;
+ }
Why is that ?
A guest context switch will involve setting cr0.ts and possibly
issuing clts after the fpu is first used:
_switch_to()
unlazy_fpu()
stts()
So we will get an exit on cr0 writes on every guest context switch
until the fpu is loaded. vmx avoids this by allowing writes that
don't change important bits to proceed.
Hmm, I see the write is conditional, so it may not be as bad as
that. We'll have to test other guests to make sure they all do
conditional stts().
It can be done. If we see a cr0 write that doesn't change cr0, we can
assume it's an stts(), load the guest fpu, and turn on selective cr0
intercepts. So the net effect will be:
- guests which only stts() when cr0.ts is clear will behave much like vmx
- guests which stts() unconditionally will see one extra exit and then
load the fpu, like current behaviour
The only tricky bit is guests with cr0.wp=0 when npt is disabled, we
have to keep cr0 intercepts there. But that's fine.
I'll write a patch.
--
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.
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