Re: [kvm-unit-tests PATCH] x86: Use "-cpu host" for PCID tests

[Sean Christopherson <sean.j.christopherson@intel.com>]

On Wed, Feb 05, 2020 at 04:26:40PM +0100, Paolo Bonzini wrote:
> On 04/02/20 20:48, Sean Christopherson wrote:
> > Use the host CPU model for the PCID tests to allow testing the various
> > combinations of PCID and INVPCID enabled/disabled without having to
> > manually change the kvm-unit-tests command line.  I.e. give users the
> > option of changing the command line *OR* running on a (virtual) CPU
> > with or without PCID and/or INVPCID.
> 
> I don't understand. :)

I was trying to test a change in the code that clears INVPCID in guest's
CPUID if PCID isn't supported.  To do that, I ran the PCID unit test in L1,
using L0 Qemu to hide PCID and/or INVPCID from L1 to iterate over the four
combinations of PCID and INVPCID being enabled/disabled.  But the test in
L1 never exercised INVPCID=y because unittest.cfg runs with the test with
"-cpu=qemu64,+pcid".

By using qemu64, the only way to test INVPCID=y is to manually run the test
a different "-cpu..." command.  The idea behind "-cpu=host" is to enable
running different combinations of the test in L1 by hiding features from L1
instead of by running the test with different commands.

In other words, if I create an L1 with PCID=y and INVPCID=y, I expect that
running the as-configured PCID unit tests would actually test PCID=Y and
INVPCID=y.