From: Sean Christopherson <sean.j.christopherson@intel.com>
To: Jim Mattson <jmattson@google.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>,
Vitaly Kuznetsov <vkuznets@redhat.com>,
Wanpeng Li <wanpengli@tencent.com>,
Joerg Roedel <joro@8bytes.org>, kvm list <kvm@vger.kernel.org>,
LKML <linux-kernel@vger.kernel.org>, Pu Wen <puwen@hygon.cn>
Subject: Re: [PATCH v2 4/7] KVM: x86: Fix CPUID range checks for Hypervisor and Centaur classes
Date: Thu, 5 Mar 2020 13:51:50 -0800 [thread overview]
Message-ID: <20200305215149.GS11500@linux.intel.com> (raw)
In-Reply-To: <CALMp9eRxdGj0DL0_g-an0YC+gTMcWcSk7=md=k4-8S0Zcankbg@mail.gmail.com>
On Thu, Mar 05, 2020 at 01:10:57PM -0800, Jim Mattson wrote:
> On Thu, Mar 5, 2020 at 11:25 AM Sean Christopherson
> <sean.j.christopherson@intel.com> wrote:
> >
> > On Thu, Mar 05, 2020 at 10:43:51AM -0800, Jim Mattson wrote:
> > > On Wed, Mar 4, 2020 at 5:34 PM Sean Christopherson
> > > <sean.j.christopherson@intel.com> wrote:
> > > >
> > > > Rework the masking in the out-of-range CPUID logic to handle the
> > > > Hypervisor sub-classes, as well as the Centaur class if the guest
> > > > virtual CPU vendor is Centaur.
> > > >
> > > > Masking against 0x80000000 only handles basic and extended leafs, which
> > > > results in Hypervisor range checks being performed against the basic
> > > > CPUID class, and Centuar range checks being performed against the
> > > > Extended class. E.g. if CPUID.0x40000000.EAX returns 0x4000000A and
> > > > there is no entry for CPUID.0x40000006, then function 0x40000006 would
> > > > be incorrectly reported as out of bounds.
> > > >
> > > > While there is no official definition of what constitutes a class, the
> > > > convention established for Hypervisor classes effectively uses bits 31:8
> > > > as the mask by virtue of checking for different bases in increments of
> > > > 0x100, e.g. KVM advertises its CPUID functions starting at 0x40000100
> > > > when HyperV features are advertised at the default base of 0x40000000.
> > > >
> > > > The bad range check doesn't cause functional problems for any known VMM
> > > > because out-of-range semantics only come into play if the exact entry
> > > > isn't found, and VMMs either support a very limited Hypervisor range,
> > > > e.g. the official KVM range is 0x40000000-0x40000001 (effectively no
> > > > room for undefined leafs) or explicitly defines gaps to be zero, e.g.
> > > > Qemu explicitly creates zeroed entries up to the Cenatur and Hypervisor
> > > > limits (the latter comes into play when providing HyperV features).
> > > >
> > > > 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.
> > > >
> > > > Note, documentation of Centaur/VIA CPUs is hard to come by. Designating
> > > > 0xc0000000 - 0xcfffffff as the Centaur class is a best guess as to the
> > > > behavior of a real Centaur/VIA CPU.
> > >
> > > Don't forget Transmeta's CPUID range at 0x80860000 through 0x8086FFFF!
> >
> > Hmm, is it actually needed here? KVM doesn't advertise support for that
> > range in KVM_GET_SUPPORTED_CPUID.
> > That's also why I limited the Centaur
> > range to vendor==CENTAUR, as KVM_GET_SUPPORTED_CPUID enumerates the
> > Centaur range if and only if the host CPU is Centaur.
Actually, I take this part of that comment back. I limited this to
vendor==CENTAUR so that it wouldn't conflict with an actual Intel CPU if
Intel adds a CPUID leaf in the 0xc0000000 range.
> Ah. So cross-vendor CPUID specifications are not supported?
Cross-vendor CPUID is sort of allowed? E.g. this plays nice with creating
a Centaur CPU on an Intel platform. My interpretation of GET_SUPPORTED...
is that KVM won't prevent enumerating what you want in CPUID, but it only
promises to correctly support select leafs.
next prev parent reply other threads:[~2020-03-05 21:51 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-03-05 1:34 [PATCH v2 0/7] KVM: x86: CPUID emulation and tracing fixes Sean Christopherson
2020-03-05 1:34 ` [PATCH v2 1/7] KVM: x86: Trace the original requested CPUID function in kvm_cpuid() Sean Christopherson
2020-03-05 17:50 ` Jim Mattson
2020-03-05 1:34 ` [PATCH v2 2/7] KVM: x86: Add helpers to perform CPUID-based guest vendor check Sean Christopherson
2020-03-05 3:48 ` Xiaoyao Li
2020-03-05 19:58 ` Sean Christopherson
2020-03-05 18:07 ` Jim Mattson
2020-03-05 1:34 ` [PATCH v2 3/7] KVM x86: Extend AMD specific guest behavior to Hygon virtual CPUs Sean Christopherson
2020-03-05 1:34 ` [PATCH v2 4/7] KVM: x86: Fix CPUID range checks for Hypervisor and Centaur classes Sean Christopherson
2020-03-05 18:43 ` Jim Mattson
2020-03-05 19:25 ` Sean Christopherson
2020-03-05 21:10 ` Jim Mattson
2020-03-05 21:51 ` Sean Christopherson [this message]
2020-03-06 9:03 ` Paolo Bonzini
2020-03-10 17:10 ` Sean Christopherson
2020-03-10 17:23 ` Jim Mattson
2020-03-05 1:34 ` [PATCH v2 5/7] KVM: x86: Add build-time assertions on validity of vendor strings Sean Christopherson
2020-03-05 1:34 ` [PATCH v2 6/7] KVM: x86: Refactor out-of-range logic to contain the madness Sean Christopherson
2020-03-05 1:34 ` [PATCH v2 7/7] KVM: x86: Refactor kvm_cpuid() param that controls out-of-range logic Sean Christopherson
2020-03-05 16:42 ` [PATCH v2 0/7] KVM: x86: CPUID emulation and tracing fixes Paolo Bonzini
2020-03-05 17:12 ` Sean Christopherson
2020-03-06 8:45 ` Paolo Bonzini
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