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

From: Like Xu
Date: Thu Apr 25 2019 - 21:30:35 EST

On 2019/4/26 0:28, Xiaoyao Li wrote:
On Thu, 2019-04-25 at 23:33 +0800, Like Xu wrote:
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).

You mean entry->eax >= 0x1f, and CPUID.1F leaf doesn't exist.
In this case, cpuid_ecx(0x1f) must be zero.

You should the descripton of CPUID instruction in SDM. It says:

If a value entered for CPUID.EAX is less than or equal to the maximum input
value and the leaf is not supported on that processor then 0 is returned in
all the registers.

It's true.


I can tell you why the cpuid.1f.exc return 0x00000064 in your machines.
That's the value of leaf 0x16, you can check the output of cpuid.0x0_eax, it
should be 0x00000016.

I have to mention in this case, the cpuid.1f.ecx[31:8] is 0 as well.


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).

I think it must be something wrong with your CLX-AP.
I tested on my CLX-AP machine too, the cpuid.0x0_eax is 0x000000001f.

You are suggesting that we should give up exposing cpuid.1f on this strange machine and it's not **practical**.

One of my CLX-AP machines is normal, but there is such another machine with a bad BIOS configuration. We are supposed to take advantage of the real hardware support instead of ignoring it just because of the cpuid.0.eax limit.



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?

There are two things I want to state:

1. using cpuid_ecx(0x1f) to check the existence of leaf 1f is absolutely wrong.

Using the return value of E{A,B,C,D}X of cpuid leaf *N* to check the existence
of leaf *N* is totally wrong. We need first to ensure that cpuid.0_eax >= *N*,
that's the reason why we need cpuid.0_eax.

Specifically, about cpuid leaf 1f, in page 3-222 Vol.2A of latest SDM publish on
January 2019, the description of Input EAX = 1FH is:

When CPUID executes with EAX set to 1FH, the processor returns information
about extended topology enumeration data. Software must detect the presence
of CPUID leaf 1FH by verifying (a) the highest leaf index supported by CPUID
is >= 1FH, and (b) CPUID.1FH:EBX[15:0] reports a non-zero value.

It's true and it's the official right way to check EBX rather than ECX. Nice move, thanks xiaoyao.

However we could do better considering all possible output of cpuid.0.eax and cpuid.1f and my proposal for whether or not CPUID.1F exists is to check cpuid.1f.ecx[31:8] practically as host check does.

I don't insist on it and what do you think, Sean?


2. Checking in descending manner is better that increasing manner.

In increasing manner, we have to check from the smallest one by one, there will
be some useless check for the smaller one.

You see the uselessness and I see the necessity.
The number of checks time is not critical.

However, in descending manner, once it finds the supported maxmium one, there is
no need to check the smaller leaf.

It's not supposed to finds the maxmium one but the minimum value
just recall the purpose of why we apply min().

The descending if_else flow would be broken if kvm does not want to expose 0x1f but a expected value smaller than 0x1f but larger than 0x14.

We may add this kind of module parameter like f_intel_pt to apply min() again and if so, an increasing manner helps a lot.

The descending flat flow couldn't reduce the number of checks time
and a temp value for original eax is introduced unavoidably.



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. :-)