Re: Found workaround/fix for ntp on AMD systems with PCI passthrough

["Radim Krčmář" <rkrcmar@redhat.com>]

2017-10-25 07:42+0200, Paolo Bonzini:
> On 24/10/2017 23:50, geoff@hostfission.com wrote:
> > In svm.c, by just changing the line in `init_vmcb` that reads:
> > 
> >    save->g_pat = svm->vcpu.arch.pat;
> > 
> > To:
> > 
> >    save->g_pat = 0x0606060606060606;
> > 
> > The problem is resolved. From what I understand this is setting a
> > MTTR value that enables Write Back (WB).
> 
> That's cool, you certainly are onto something.  Currently, SVM is
> disregarding the guest PAT setting (PA0=PA4=WB, PA1=PA5=WT, PA2=PA6=UC-,
> PA3=UC).  The guest might be using a different setting so you're
> getting slow accesses (UC- or UC, i.e. uncacheable) instead of fast
> accesses (WB or WC, respectively writeback and write combining).
> 
> It would be great if you could proceed with the following tests:
> 
> 1) see if this patch has any effect
> 
> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> index af256b786a70..b2e4b912f053 100644
> --- a/arch/x86/kvm/svm.c
> +++ b/arch/x86/kvm/svm.c
> @@ -3626,6 +3626,12 @@ static int svm_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr)
>  	u32 ecx = msr->index;
>  	u64 data = msr->data;
>  	switch (ecx) {
> +	case MSR_IA32_CR_PAT:
> +		if (!kvm_mtrr_valid(vcpu, MSR_IA32_CR_PAT, data))
> +			return 1;
> +		vcpu->arch.pat = data;
> +		svm->vmcb->save.g_pat = data;

Great progress!  SVM might cache the value and adding

+		mark_dirty(svm->vmcb, VMCB_NPT);

here should result in the same behavior as doing (2).

> +		break;
>  	case MSR_IA32_TSC:
>  		kvm_write_tsc(vcpu, msr);
>  		break;
> 
> 2) if it doesn't, add a printk("%#016lx", data); to the patch and get the
> last value written by the guest.  Hard-code it in the "save->g_pat = ..."
> line where you've been using 0x0606060606060606 successfully.  Test that
> things work (though they should still be slow).
> 
> 3) starting from the rightmost byte, change one byte to 0x06, test that
> and see if things get fast.  For each byte you change, take a note of the
> full value and whether things are slow or fast.
> 
> Thank you very much!
> 
> Paolo