linux-kernel - Re: [PATCH v5 09/19] KVM:x86: Make guest supervisor states as non-XSAVE managed

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Date:   Thu, 10 Aug 2023 08:37:48 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Dave Hansen <dave.hansen@...el.com>,
        Weijiang Yang <weijiang.yang@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>, peterz@...radead.org,
        Chao Gao <chao.gao@...el.com>, john.allen@....com,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        rick.p.edgecombe@...el.com, binbin.wu@...ux.intel.com
Subject: Re: [PATCH v5 09/19] KVM:x86: Make guest supervisor states as
 non-XSAVE managed

On Thu, Aug 10, 2023, Paolo Bonzini wrote:
> On 8/10/23 16:29, Dave Hansen wrote:
> > On 8/10/23 02:29, Yang, Weijiang wrote:
> > ...
> > > When KVM enumerates shadow stack support for guest in CPUID(0x7,
> > > 0).ECX[bit7], architecturally it claims both SS user and supervisor
> > > mode are supported. Although the latter is not supported in Linux,
> > > but in virtualization world, the guest OS could be non-Linux system,
> > > so KVM supervisor state support is necessary in this case.
> > 
> > What actual OSes need this support?
> 
> I think Xen could use it when running nested.  But KVM cannot expose support
> for CET in CPUID, and at the same time fake support for
> MSR_IA32_PL{0,1,2}_SSP (e.g. inject a #GP if it's ever written to a nonzero
> value).
> 
> I suppose we could invent our own paravirtualized CPUID bit for "supervisor
> IBT works but supervisor SHSTK doesn't".  Linux could check that but I don't
> think it's a good idea.
> 
> So... do, or do not.  There is no try. :)

> > I want to hear more about who is going to use CET_S state under KVM in
> > practice.  I don't want to touch it if this is some kind of purely
> > academic exercise.  But it's also silly to hack some kind of temporary
> > solution into KVM that we'll rip out in a year when real supervisor
> > shadow stack support comes along.

As Paolo alluded to, this is about KVM faithfully emulating the architecture.
There is no combination of CPUID bits that allows KVM to advertise SHSTK for
userspace without advertising SHSTK for supervisor.

Whether or not there are any users in the short term is unfortunately irrelevant
from KVM's perspective.