Re: [PATCH] KVM: x86: Add Intel CPUID.1F cpuid emulation support

[Like Xu]

On 2019/4/25 22:19, Sean Christopherson wrote:
> On Thu, Apr 25, 2019 at 03:07:35PM +0800, Like Xu wrote:
> > On 2019/4/25 14:30, Xiaoyao Li wrote:
> > > > > Besides, the problem of simply using cpuid_exc(0x1f) in Sean's codes is
> > > > > that we cannot assmue the reserved bits 31:16 of ECX is always 0 for the
> > > > > future generation.
>
> It doesn't matter if future CPUs use 31:16 for other things since this
> code only cares about whether or not CPUID.1F exists.  The check
> entry->eax >= 0x1f ensures that CPUID.1F will return zeros if the leaf
> does *NOT* exist, ergo the check against CPUID.1F.ECX only needs to
> look for a non-zero value.  CPUID.1F.ECX is the logical choice for
> detecting support because it is guaranteed to be non-zero if the leaf
> actually exists (because bits 15:8 will be non-zero).  Whether or not
> bits 31:16 are non-zero is irrelevant.

Here is a case:

one future CPU may have "cpuid.0.eax > 0x1f" but not use multi-chip 
packaging technology thus its CPUID.1F leaf **could** not exist to 
expose multi-die info and it still use cpuid.0B leaf.

So the entry->eax >= 0x1f check is true and cpuid_ecx(0x1f) check is 
true as well due to default return value. (one of my machines for 
cpuid.1f.ecx would return 0x00000064 without cpuid.1f support).

When we only cares about whether or not CPUID.1F exists,
we may need (cpuid_ecx(0x1f) & 0x0000ff00) != 0 or just follow what host 
check_extended_topology_leaf() does.

> > > > It's true cause the statement in public spec is not "Reserved = 0" but
> > > > "Bits 31 - 16: Reserved".
> > > >
> > > > > In my opinion, Sean's codes is OK and much simple and clear.
> > > >
> > > > If the host cpuid.0.eax is greater than 0x1f but actually it doesn't
> > > > have multi-chip packaging technology and we may want to expose
> > > > entry->eax to some value smaller than 0x1f but greater than 0x14, much
> > > > effort needs to apply on Sean's code.
> > > >
> > > > My improvement is good to overwrite cpuid.0.eax in future usage
> > > >from the perspective of kvm feature setting not just from value check.
> > >
> > > Alright, there is something wrong in your code that you haven't realised.
> > >
> > > When you do
> > > 	entry->eax = min(entry->eax, (u32)(f_intel_pt ? 0x14 : 0xd));
> > >
> > > it changes the entry->eax if entry->eax > 0x14. So you cannot directly use
> > > cpuid_ecx(0x1f). At least, you need to cache the value of entry->eax, like:
> > > 	
> > > 	u32 max_leaf = entry->eax;
> > > 	entry->eax = min(entry->eax, (u32)(f_intel_pt ? 0x14 : 0xd));
> > > 	
> > > 	//...leaf between 0x14 and 0x1f
> > > 	
> > > 	if (max_leaf >= 0x1f && (cpuid_ecx(0x1f) & 0x0000ff00))
> > >   		entry->eax = 0x1f;
> >
> > The cache value make no sense on this.
>
> Xiaoyao is pointing out that by limiting entry->eax to 0x14 (or 0xd), then
> it can't be used to detect support for CPUID.1F since KVM will have lost
> the required information.

The point is that its existence does not really depend on cpuid.0.eax> = 
0x1f. My test machine (CLX-AP, two die in one package) has cpuid.0.eax = 
0x16 but does have CPUID.1F support (a bit strange on BIOS).

> > > However, handling in increasing order in totally wrong. Since it's to report the
> > > max the leaf supported, we should handle in descending order, which is what Sean
> > > does.
> >
> > There is no need to check "entry->eax >= 0x1f" before "setting entry->eax =
> > 0x1f" if and only if cpuid_ecx(0x1f) meets requirements.
>
> entry->eax absolutely needs to be checked, otherwise you have no idea what
> CPUID leaf is actually being consumed.  For example, a CPU with a maximum
> basic CPUID leaf of 0xB will report information that is indistinguishable
> from the actual CPUID.1F, i.e. KVM will incorrectly think the CPU supports
> CPUID.1F regardless of what heuristic is used to detect a "valid" CPUID.1F
> if it only looks at the result of CPUID.1F.

Based on what I mentioned, just reconsider this proposal:

case 0:
    entry->eax = min(entry->eax, (u32)(f_intel_pt ? 0x14 : 0xd));
    if ((cpuid_ecx(0x1f) & 0x0000ff00)) != 0)
        entry->eax = 0x1f;

It increases the cpuid.0.ecx value along with the minimized feature 
requirement in a natural and readable way. Why don't we design a little 
bit future ahead of time?

> > An increasing manner helps to overwrite this value on demand in a flat code
> > flow (easy to understand and maintain) not an if-else_if-else flow.
>
> Maybe in the future the code will need to be refactored as more cases are
> added, but for now an if-else is quite readable.  Worry about the future
> when it happens. :-)