On Tue, Mar 03, 2020 at 09:42:42AM -0800, Jim Mattson wrote:
> Unfathomable was the wrong word.
I dunno, one could argue that the behavior of Intel CPUs for CPUID is
unfathomable and I was just trying to follow suit :-D
> I can see what you're trying to do. I
> just don't think it's defensible. I suspect that Intel CPU architects
> will be surprised and disappointed to find that the maximum effective
> value of CPUID.0H:EAX is now 255, and that they have to define
> CPUID.100H:EAX as the "maximum leaf between 100H and 1FFH" if they
> want to define any leaves between 100H and 1FFH.
Hmm, ya, I agree that applying a 0xffffff00 mask to all classes of CPUID
ranges is straight up wrong.
> Furthermore, AMD has only ceded 4000_0000h through 4000_00FFh to
> hypervisors, so kvm's use of 40000100H through 400001FFH appears to be
> a land grab, akin to VIA's unilateral grab of the C0000000H leaves.
> Admittedly, one could argue that the 40000000H leaves are not AMD's to
> apportion, since AMD and Intel appear to have reached a detente by
> splitting the available space down the middle. Intel, who seems to be
> the recognized authority for this range, declares the entire range
> from 40000000H through 4FFFFFFFH to be invalid. Make of that what you
> will.
>
> In any event, no one has ever documented what's supposed to happen if
> you leave gaps in the 4xxxxxxxH range when defining synthesized CPUID
> leaves under kvm.
Probably stating the obvious, but for me, the least suprising thing is for
such leafs to output zeros. It also feels safer, e.g. a guest that's
querying hypervisor support is less likely to be led astray by all zeros
than by a random feature bits being set.
What about something like this? Along with a comment and documentation...
static bool cpuid_function_in_range(struct kvm_vcpu *vcpu, u32 function)
{
struct kvm_cpuid_entry2 *max;
if (function >= 0x40000000 && function <= 0x4fffffff)
max = kvm_find_cpuid_entry(vcpu, function & 0xffffff00, 0);
else
max = kvm_find_cpuid_entry(vcpu, function & 0x80000000, 0);
return max && function <= max->eax;
}
> On Mon, Mar 2, 2020 at 8:58 PM Sean Christopherson
> <sean.j.christopherson@xxxxxxxxx> wrote:
> >
> > On Mon, Mar 02, 2020 at 08:25:31PM -0800, Jim Mattson wrote:
> > > On Mon, Mar 2, 2020 at 7:25 PM Jim Mattson <jmattson@xxxxxxxxxx> wrote:
> > > >
> > > > On Mon, Mar 2, 2020 at 11:57 AM Sean Christopherson
> > > > <sean.j.christopherson@xxxxxxxxx> wrote:
> > > >
> > > > > The bad behavior can be visually confirmed by dumping CPUID output in
> > > > > the guest when running Qemu with a stable TSC, as Qemu extends the limit
> > > > > of range 0x40000000 to 0x40000010 to advertise VMware's cpuid_freq,
> > > > > without defining zeroed entries for 0x40000002 - 0x4000000f.
> > > >
> > > > I think it could be reasonably argued that this is a userspace bug.
> > > > Clearly, when userspace explicitly supplies the results for a leaf,
> > > > those results override the default CPUID values for that leaf. But I
> > > > haven't seen it documented anywhere that leaves *not* explicitly
> > > > supplied by userspace will override the default CPUID values, just
> > > > because they happen to appear in some magic range.
> > >
> > > In fact, the more I think about it, the original change is correct, at
> > > least in this regard. Your "fix" introduces undocumented and
> > > unfathomable behavior.
> >
> > Heh, the takeaway from this is that whatever we decide on needs to be
> > documented somewhere :-)
> >
> > I wouldn't say it's unfathomable, conceptually it seems like the intent
> > of the hypervisor range was to mimic the basic and extended ranges. The
> > whole thing is arbitrary behavior. Of course if Intel CPUs would just
> > return 0s on undefined leafs it would be a lot less arbitrary :-)
> >
> > Anyways, I don't have a strong opinion on whether this patch stays or goes.
